fix(integrations): Add mechanism parameter to captured exceptions in MCP and Langchain

[harryautomazione]

This PR fixes issue #5242 by adding the explicit mechanism parameter with {"handled": False} to captured exceptions in both mcp.py and langchain.py.

🔍 [MCP]

In _handler_wrapper, replaced generic capture_exception with explicit event_from_exception building to provide the mechanism data.

🦜 [LangChain]

In _handle_error, updated capture_exception to use event_from_exception allowing explicit mechanism payload allocation.

Verified locally with simulated error streams proving the existence of correct mechanism keys.

[github-actions]

Semver Impact of This PR

🟢 Patch (bug fixes)

📋 Changelog Preview

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Entries from this PR are highlighted with a left border (blockquote style).

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New Features ✨

• (asgi) Add option to disable suppressing chained exceptions by alexander-alderman-webb in #5714
• (logging) Separate ignore lists for events/breadcrumbs and sentry logs by sl0thentr0py in #5698

Bug Fixes 🐛

Anthropic

• Set exception info on streaming span when applicable by alexander-alderman-webb in #5683
• Patch AsyncStream.close() and AsyncMessageStream.close() to finish spans by alexander-alderman-webb in #5675
• Patch Stream.close() and MessageStream.close() to finish spans by alexander-alderman-webb in #5674

Other

• (integrations) Add mechanism parameter to captured exceptions in MCP and Langchain by harryautomazione in #5732

Documentation 📚

• Add note on AI PRs to CONTRIBUTING.md by sentrivana in #5696

Internal Changes 🔧

• Add -latest alias for each integration test suite by sentrivana in #5706
• Use date-based branch names for toxgen PRs by sentrivana in #5704
• 🤖 Update test matrix with new releases (03/19) by github-actions in #5703
• Add client report tests for span streaming by sentrivana in #5677

Other

• Update CHANGELOG.md by sentrivana in #5685

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