[bug-hunter] OR version constraint picks newer stack version due lexicographic compare

## Impact
Buildkite can select the wrong Kibana stack version when a package manifest uses an OR constraint with two-digit minors (for example `8.10.0||8.9.0`). This makes CI run against a newer/incorrect stack version and can produce misleading pass/fail results for package validation.

## Reproduction Steps
1. Run the following reproduction script from the repository root:

```python
python3 - <<'PY'
import importlib.util, pathlib
from packaging.version import Version

p = pathlib.Path('.buildkite/scripts/find_oldest_supported_version.py')
spec = importlib.util.spec_from_file_location('finder', p)
mod = importlib.util.module_from_spec(spec)
spec.loader.exec_module(mod)

# Control available versions and avoid network calls
mod.fetch_version = lambda: {'versions': ['8.9.0', '8.10.0'], 'aliases': []}

condition = '8.10.0||8.9.0'
actual = mod.find_oldest_supported_version(condition)
expected = str(min([Version('8.10.0'), Version('8.9.0')]))

print(f'condition={condition}')
print(f'expected={expected}')
print(f'actual={actual}')
assert actual == expected, f'BUG: expected {expected}, got {actual}'
PY
```

2. Observe output:

```text
condition=8.10.0||8.9.0
expected=8.9.0
actual=8.10.0
Traceback (most recent call last):
File "", line 16, in
AssertionError: BUG: expected 8.9.0, got 8.10.0
```

## Expected vs Actual
**Expected:** The oldest semantic version in `8.10.0||8.9.0` is `8.9.0`.

**Actual:** The script returns `8.10.0` because it compares strings lexicographically.

## Failing Test
```python
import importlib.util, pathlib
from packaging.version import Version

p = pathlib.Path('.buildkite/scripts/find_oldest_supported_version.py')
spec = importlib.util.spec_from_file_location('finder', p)
mod = importlib.util.module_from_spec(spec)
spec.loader.exec_module(mod)

mod.fetch_version = lambda: {'versions': ['8.9.0', '8.10.0'], 'aliases': []}

condition = '8.10.0||8.9.0'
actual = mod.find_oldest_supported_version(condition)
expected = str(min([Version('8.10.0'), Version('8.9.0')]))
assert actual == expected, f'BUG: expected {expected}, got {actual}'
```

## Evidence
- `.buildkite/scripts/find_oldest_supported_version.py` line 77 compares candidates with `candidate < result`, which is lexicographic string ordering instead of semantic version ordering.
- `.buildkite/scripts/common.sh` lines 450-457 invoke this script to choose stack version in CI, so the bug directly affects pipeline behavior.
- Searched for existing tracking (`find_oldest_supported_version`, `oldest supported version`, related PR/issue queries) and did not find an open issue for this specific lexicographic OR-order bug.

> [!NOTE]
>
> 🔒 Integrity filter blocked 1 item
>
> The following item were blocked because they don't meet the GitHub integrity level.
>
> - [#6945](https://github.com/elastic/integrations/pull/6945) `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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