fix(transport): clear stale optional cA2A metadata on reused SDK messages

[miyannishar]

What

attach_to_sdk_message now attaches cA2A metadata against the SDK message's current metadata instead of a throwaway {}, so an absent optional key (sealed_payload, caller_offer, holder_proof) is cleared when a Message is reused across calls. Adds round-trip tests covering all three optional keys.

Why

The bridge mutates an SDK Message in place. It attached via a2a_adapter.attach_ca2a_metadata({}, request), but the dict adapter clears an absent optional key with meta.pop(KEY_..., None) — run against {} those pops are no-ops, so only present keys were overlaid onto the message's stale metadata and an earlier request's optional key survived. The dict adapter itself is correct; only the SDK bridge diverged. Attaching against the real current metadata restores parity with the dict adapter.

Closes #166.

Security impact

Fixes a cross-request payload-confusion bug on the sealed-channel carrier: a reused SDK Message could ship an earlier task's sealed_payload (and likewise caller_offer / holder_proof). With holder-proof enforcement it fails closed (HolderProofInvalid); in advisory/unenforced modes the callee would open the stale ciphertext as this task's payload. The fix ensures reused messages reflect only the current request.

Test plan

• pytest passes (tests/unit/ + tests/conformance/: 604 passed, 2 skipped; 3 new tests red before the fix, green after)
• ruff check passes
• mypy passes (ruff format --check and bandit also clean)
• Manual test performed

Repro before the fix (against the real a2a-sdk):

from a2a.types import Message
from ca2a_runtime.transport import a2a_sdk
from tests.unit.test_a2a_sdk_bridge import _request
msg = Message(message_id="m1")
a2a_sdk.attach_to_sdk_message(msg, _request(sealed_payload=b"SECRET"))
a2a_sdk.attach_to_sdk_message(msg, _request())
print(a2a_sdk.parse_sdk_message(msg).sealed_payload)  # was b"SECRET", now None

DCO sign-off

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[carloshvp]

Reviewed commit 1e038a9. No blocking findings.

The bridge now applies the dict adapter to the current metadata, so absent sealed_payload, caller_offer, and holder_proof fields are removed on reuse while unrelated metadata is preserved. This fixes the stale cross-request carrier data without changing verification policy.

Validation in an isolated checkout on Python 3.13 with the real a2a-sdk 1.1.2: unit and conformance suites passed (604 passed, 2 skipped); ruff check, ruff format --check, mypy, and Bandit passed. All three new regression tests fail against the parent implementation. An additional eight-combination optional-field matrix, repeated three times per combination, verified clearing, replacement, unrelated metadata preservation, routing preservation, and extension idempotence.

Approval covers this exact head. The hosted CI matrix is not present in the current check rollup; local validation does not replace remaining repository gates.

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