docs(adcp): fix a2a-sdk 1.0.x symbol guidance in v3→v4 migration guide

[bokelley]

Closes #514

Summary

MIGRATION_v3_to_v4.md told adopters that a2a.utils.errors.ServerError migrates to a2a.types.A2AError and a2a.types.DataPart migrates to a2a.types.MessagePart. Neither a2a.types.A2AError nor a2a.types.MessagePart exist in a2a-sdk==1.0.1 (the version this SDK pins). A real adopter (salesagent v3→v4 migration) gated an entire module behind A2A_LEGACY_AVAILABLE and skipped 6 test modules because the guide pointed at dead symbols.

This PR replaces the one-liner "see release notes" with two concrete subsections:

1. Hand-rolled a2a-sdk 0.3 servers — no mechanical migration path; the recommended path is to delete the hand-rolled server and use adcp.server.serve(transport="a2a").
2. Direct a2a type imports — correct symbol table: DataPart/TextPart → a2a.types.Part; Part(root=...) wrapper → Part directly; ServerError → a2a.utils.errors.A2AError; and explicit note that a2a.types.A2AError and a2a.types.MessagePart do not exist.

Also corrects the version specifier in the prose from >=1.0.0 to >=1.0.1,<1.0.2 to match the actual pin.

What tested

• Docs-only change — no Python build or pytest run required per triage protocol
• A2AError path verified against src/adcp/server/translate.py:32
• DataPart/TextPart/Part semantics verified against tests/a2a_compat_shim.py:116–129
• a2a.server.routes + a2a.server.agent_execution paths verified against src/adcp/server/a2a_server.py:28–37
• Version pin >=1.0.1,<1.0.2 and regression comment verified in pyproject.toml:51–57

Pre-PR review

• code-reviewer: approved — all factual claims verified against codebase, no blockers; nit on version string staleness (cross-reference to pyproject.toml comment is accurate and sufficient)
• docs-expert: approved — audience fit correct, acceptance criterion met; nit on heading framing (noted in PR body rather than changed — "no mechanical migration path" is accurate and intentionally direct)

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_015ztuszM5hAJG7N5NcA9acB

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