fix(a2a): preserve inbound message in task writeback

[RuniThomsen]

Summary

• persist the inbound A2A Message as the receiver-owned Task.history child
• preserve referenceTaskIds, governed metadata, extensions, parts, and correlation IDs
• stamp the receiver-minted taskId without mutating the caller request
• return the write-back record in both non-holding SendMessage receipts and the first SendStreamingMessage Task frame
• drop caller-only top-level keys so strict Task ProtoJSON remains valid

Why

NousResearch#727 requires one seat dispatch to produce/attach a visible task row without a second write. The sender-side coupling needs the receiver's first receipt to carry the authoritative Task plus the original Message as its child. Hermes previously minted the Task but discarded that Message from Task.history.

Verification

• TDD RED observed for TaskStore history, non-holding receipt, and streaming first frame
• 157 passed, 18 deselected for test_a2a_plugin.py + test_a2a_phase23.py
• focused streaming integration: 1 passed
• py_compile and git diff --check pass
• broad integration has one unrelated anti-loop expectation that already fails on exact base d048b78480 because that test still expects holding completed instead of the new non-holding working receipt

[RuniThomsen]

CI classification: the fork workflows failed before execution. Each failing detector/OSV job reports an empty runner name, zero steps, and no logs; all code/lint/test jobs were skipped. This is infrastructure red, not a candidate test failure.

Executed evidence on exact base d048b78480:

• A2A plugin/phase focused suite: 157 passed, 18 integration rows deselected.
• feat: cognitive memory system - semantic recall, encoding & forgetting NousResearch/hermes-agent#727 streaming write-back integration: 1 passed.
• py_compile and git diff --check: pass.
• One broad anti-loop integration expectation is non-green on both candidate and exact base because base d048 changed SendMessage to return WORKING before completion while that old test still expects immediate COMPLETED.