fix(ci): add --extra dev to pre-commit mypy hook to include types-protobuf stubs

[bokelley]

Closes #411

Without --extra dev, uv run mypy does not activate [project.optional-dependencies] dev, which contains types-protobuf. The missing stubs cause 96 false-positive errors from a2a/google.protobuf imports in client.py and protocols/a2a.py. CI passes cleanly because it installs dev extras explicitly (pip install -e ".[dev]"). Adding --extra dev makes the pre-commit environment match CI so the hook no longer needs routine SKIP=mypy bypasses.

What was tested:

• uv run --extra dev pytest tests/ -q (excluding the pre-existing network-bound signing test test_real_tls_handshake_still_validates_hostname): 3188 passed, 26 skipped, 1 xfailed
• Pre-existing signing-test failure confirmed on main before this change (unrelated: real TLS call to example.com returns 403 in the sandbox)
• YAML syntax validated

Pre-PR review:

• code-reviewer: approved — 1 nit: notes types-protobuf is duplicated in both [project.optional-dependencies] dev and [dependency-groups] dev; --extra dev is the correct flag to activate the former (where CI installs from)
• dx-expert: approved — nit: --group dev is the more semantically idiomatic uv flag since uv add --dev writes to [dependency-groups]; both flags work today because the stub is duplicated in both sections; inline comment is sufficient

Triage-managed PR. This bot does not currently iterate on
review comments or PR conversation threads (only on the source
issue). To unblock:

• Push fixup commits directly: gh pr checkout <num> →
  fix → push.
• Or re-trigger: comment /triage execute on the source
  issue.

See adcp#3121
for context.

Session: https://claude.ai/code/session_012s2eENfKoeaeeHUyV7Rd9y

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