[bug-hunter] Changelog link checker rejects valid quoted PR links

## Impact
PRs that add a valid quoted changelog PR link (for example `link: "https://github.com/elastic/integrations/pull/100"`) are incorrectly marked as failures by the new Buildkite changelog-link check. This produces false CI errors and unnecessary PR annotation/comment noise.

## Reproduction Steps
1. Run this minimal repro script:

```bash
REPO=/home/runner/work/integrations/integrations
TMP=/tmp/gh-aw/agent/repro-changelog-quote
rm -rf "$TMP"
mkdir -p "$TMP"
cd "$TMP"

git init -q
git config user.name repro
git config user.email repro@example.com

cat > changelog.yml <<'EOF'
changelog:
- version: 1.0.0
changes:
- description: initial
type: enhancement
link: https://github.com/elastic/integrations/pull/99
EOF

git add changelog.yml
git commit -q -m 'base'

cat > changelog.yml <<'EOF'
changelog:
- version: 1.0.1
changes:
- description: second
type: enhancement
link: "https://github.com/elastic/integrations/pull/100"
- version: 1.0.0
changes:
- description: initial
type: enhancement
link: https://github.com/elastic/integrations/pull/99
EOF

git add changelog.yml
git commit -q -m 'add quoted link'

source "$REPO/.buildkite/scripts/check_changelog_entries.sh"
check_changelog_file HEAD~1 changelog.yml "https://github.com/elastic/integrations/pull/100"
```

## Expected vs Actual
**Expected:** quoted and unquoted identical PR URLs should be treated as equivalent and pass.

**Actual:** quoted URL is treated as different and fails:

```text
--- [changelog.yml]
ERROR: unexpected link: '"https://github.com/elastic/integrations/pull/100"'
expected: 'https://github.com/elastic/integrations/pull/100'
```

## Failing Test
```bash
#!/usr/bin/env bash
set -euo pipefail

REPO=/home/runner/work/integrations/integrations
TMP=/tmp/gh-aw/agent/repro-changelog-quote
rm -rf "$TMP"
mkdir -p "$TMP"
cd "$TMP"

git init -q
git config user.name repro
git config user.email repro@example.com

cat > changelog.yml <<'EOF'
changelog:
- version: 1.0.0
changes:
- description: initial
type: enhancement
link: https://github.com/elastic/integrations/pull/99
EOF

git add changelog.yml
git commit -q -m 'base'

cat > changelog.yml <<'EOF'
changelog:
- version: 1.0.1
changes:
- description: second
type: enhancement
link: "https://github.com/elastic/integrations/pull/100"
- version: 1.0.0
changes:
- description: initial
type: enhancement
link: https://github.com/elastic/integrations/pull/99
EOF

git add changelog.yml
git commit -q -m 'add quoted link'

source "$REPO/.buildkite/scripts/check_changelog_entries.sh"
set +e
check_changelog_file HEAD~1 changelog.yml "https://github.com/elastic/integrations/pull/100"
status=$?
set -e

# This currently fails with status=1.
echo "status=$status"
exit "$status"
```

## Evidence
- Regression introduced in commit `e385cb32f7f30319278882648a4b248b9e1e39f8` (`[CI] Add Buildkite step to validate changelog PR links (#19171)`).
- The extraction keeps surrounding quotes: `.buildkite/scripts/check_changelog_entries.sh:20-23` (`sed -E 's/^\+[[:space:]]+link:[[:space:]]*//'`).
- The comparison is strict raw string equality: `.buildkite/scripts/check_changelog_entries.sh:55`.
- Quoted PR links already exist in repository changelogs, e.g.:
- `packages/airflow_otel/changelog.yml:11`
- `packages/elastic_agent/changelog.yml:21`
- Current tests do not cover quoted PR links: `.buildkite/scripts/test_check_changelog_entries.sh` has positive/negative URL tests but no quoted-link case.

> [!NOTE]
>
> 🔒 Integrity filter blocked 13 items
>
> The following items were blocked because they don't meet the GitHub integrity level.
>
> - [#18610](https://github.com/elastic/integrations/issues/18610) `search_issues`: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".
> - [#19248](https://github.com/elastic/integrations/issues/19248) `search_issues`: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".
> - [#18288](https://github.com/elastic/integrations/issues/18288) `search_issues`: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".
> - [#17392](https://github.com/elastic/integrations/issues/17392) `search_issues`: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".
> - [#18505](https://github.com/elastic/integrations/pull/18505) `search_pull_requests`: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".
> - [#18819](https://github.com/elastic/integrations/pull/18819) `search_pull_requests`: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".
> - [#18890](https://github.com/elastic/integrations/pull/18890) `search_pull_requests`: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".
> - [#18944](https://github.com/elastic/integrations/pull/18944) `search_pull_requests`: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".
> - [#18135](https://github.com/elastic/integrations/pull/18135) `search_pull_requests`: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".
> - [#17231](https://github.com/elastic/integrations/pull/17231) `search_pull_requests`: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".
> - [#17886](https://github.com/elastic/integrations/pull/17886) `search_pull_requests`: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".
> - [#15041](https://github.com/elastic/integrations/pull/15041) `search_pull_requests`: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".
> - [#13557](https://github.com/elastic/integrations/pull/13557) `search_pull_requests`: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".
>
> To allow these resources, lower `min-integrity` in your GitHub frontmatter:
>
> ```yaml
> tools:
> github:
> min-integrity: approved # merged | approved | unapproved | none
> ```
>
>

---
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