Add Codecov coverage badge to README

[jariy17]

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:

[![codecov](https://codecov.io/gh/aws/bedrock-agentcore-sdk-python/branch/main/graph/badge.svg)](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)