fix(teams): classify group chats by conversation type

[onmax]

Summary

Fixes a:-prefixed Teams group chats being treated as DMs, so they use non-DM routing and buffered responses.

Microsoft defines conversation IDs as opaque strings and conversationType as the channel-provided conversation discriminator. The adapter therefore stores a classification override only when conversationType disagrees with the legacy ID-prefix heuristic. Existing subscription and history keys remain stable when the heuristic was already correct. Explicit group chats also bypass stale DM Graph context, and Graph-listed child threads retain the override.

Test plan

• The Teams webhook integration scenario exercises an a:-prefixed groupChat; all 12 Teams integration tests pass.
• All 94 Teams adapter and Graph tests pass, including legacy-ID stability and stale DM-context coverage.
• TypeScript and Biome checks pass for the affected packages and files.
• GitHub changeset, lint, consistency, typecheck, AI SDK compatibility, and Node 22/24 build-and-test jobs pass.
• pnpm validate was not run locally because the workspace pnpm signature guard could not verify the package-manager registry signatures.
• Vercel previews were not created because fork deployments require authorization; these statuses did not run a build.

Checklist

• All commits are signed and verified
• All commits are signed off for the DCO (git commit -s)
• pnpm validate passes
• Changeset added (or N/A — see CONTRIBUTING.md)
• Documentation updated (or N/A)

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