fix: compare in-memory task timestamps numerically

[AndreyGatsuk]

Description

Fix timestamp filtering and descending ordering in InMemoryTaskStore.list(). Comparing ToJsonString() values lexicographically reverses chronological order when fractional-second precision differs. For example, 2025-01-01T00:00:00.123Z incorrectly passes a filter whose threshold is 2025-01-01T00:00:00.123000001Z, despite being one nanosecond earlier. The same comparison also produces incorrectly ordered pages.

Compare integer nanoseconds using ToNanoseconds() for both filtering and sorting. Preserve the inclusive filter boundary, descending ID tie-breaker, and placement of tasks without timestamps after timestamped tasks.

A2A specification §5.6.1, Timestamps states: “Millisecond precision SHOULD be used where available”. It also specifies google.protobuf.Timestamp, whose representation contains seconds and nanoseconds, and permits omitted fractional seconds. Numeric comparison handles these precisions without discarding sub-millisecond information.

Add 15 regression cases covering seconds, milliseconds, microseconds, nanoseconds, equal timestamps, second boundaries, pagination, ID tie-breaking, absent timestamps/status, and timestamps before/at the Unix epoch. Before the fix, 11 cases fail and four pass; after the fix, all 35 in-memory task store tests pass.

Validation on Python 3.10 with PostgreSQL 15 and MySQL 8:

• ./scripts/lint.sh: Ruff check/format and ty passed (three existing unused-ignore warnings outside this change).
• uv run pytest: 2,092 passed, 3 xfailed, 1 xpassed. The xfail/xpass cases are already marked for [Feat]: Improve server concurrency architecture #869.
• uv run pytest --cov=src --cov-report=term-missing --cov-fail-under=88: same test results, 93.17% coverage.
• JSCPD: 2.61% duplication, below the 3% threshold.
• Built the wheel and ran tests.install_smoke in six clean environments: base, http-server, fastapi, grpc, telemetry, and sql all passed.

Local transport checks used proxy exclusions for loopback addresses. Cross-version uv --with environments were prepared before the full runs to keep dependency downloads outside server-startup timeouts.

• Follow the CONTRIBUTING guide: feature branch based on current upstream main.
• Conventional Commit title: fix: compare in-memory task timestamps numerically.
• Tests and linters pass.
• Protocol reference included above; no public API changes.

[github-actions]

🧪 Code Coverage (vs main)

⬇️ Download Full Report

	Base	PR	Delta
src/a2a/server/tasks/inmemory_task_store.py	97.06%	100.00%	🟢 +2.94%
Total	93.06%	93.09%	🟢 +0.03%

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