Add Codecov coverage badge to README

## Summary

The CI pipeline already uploads test coverage data to Codecov (in `.github/workflows/ci.yml`), but the README doesn't display a coverage badge. Adding one gives contributors a quick visual indicator of test coverage health.

## Current State

- Coverage is collected via `pytest-cov` with a 90% `fail_under` threshold
- Coverage XML is uploaded to Codecov on every CI run
- The README has badges for commit activity, issues, PRs, license, PyPI version, and Python versions — but not coverage

## Implementation

Add the Codecov badge to the existing badge row in `README.md`:

```markdown
[](https://codecov.io/gh/aws/bedrock-agentcore-sdk-python)
```

### Steps

1. Verify the Codecov project is set up at https://codecov.io/gh/aws/bedrock-agentcore-sdk-python — if not, a repo admin needs to enable Codecov integration for the repo
2. Add the badge markdown to `README.md` alongside the existing badges
3. Confirm the badge renders and shows a valid percentage

## Notes

- No code changes needed — just a README update
- If the Codecov integration isn't active yet, a repo admin needs to enable it via the Codecov GitHub App or by adding a `CODECOV_TOKEN` repository secret (Codecov v5 action supports tokenless uploads for public repos, but a token is more reliable)