fix: tighten package specifier validation in install_packages()

[notgitika]

Summary

Tightens package specifier validation in install_packages():

• Restricts the extras group in the package-name pattern to valid extra names (comma-separated identifiers) instead of allowing arbitrary characters.
• Quotes each package argument with shlex.quote() when building the pip install command so each is passed as a single literal token.

Tests

• Adds a test that extras groups with characters outside valid extra names are rejected.
• Adds a test confirming valid extras (e.g. pandas[security], celery[redis,auth]) still work.
• Updates the version-specifier test for the quoting change.

[github-actions]

⚠️ Breaking Change Warning

Found 1 potential breaking change(s) in this PR:

�[1msrc/bedrock_agentcore/tools/code_interpreter_client.py�[0m:27: VALID_PACKAGE_NAME: �[33mAttribute value was changed�[39m: re.compile('^a-zA-Z0-9?(\[.\])?(==|>=|<=|!=|~=|>|<)?[a-zA-Z0-9.]$') -> re.compile('^a-zA-Z0-9?(\[[a-zA-Z0-9._,\-]\])?(==|>=|<=|!=|~=|>|<)?[a-zA-Z0-9.]$')

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Note: This is an automated static analysis check. Some flagged changes may be intentional.
Please confirm each item is expected and, if so, add a migration note to CHANGELOG.md.

[agentcore-devx-automation]

Claude Security Review: no high-confidence findings. (run)

[notgitika]

Breaking change expected. We changed the regex