feat(session): add conformance checker and pluggable storage resolution #341

[mmpouya]

Implements steps 2 and 3 from #341
treats SessionStorage as a stable public extension point.

Changes:

Step 2 — Conformance checker (tau_agent.session.conformance)

• verify_session_storage() — a pytest-ready helper that validates any SessionStorage implementation against the storage contract: fresh storage is empty, appends are durable and repeatable, all entry types round-trip, and reopen preserves data.
• SessionStorageConformanceError — raised on the first violation with a clear message.
• conformance_entries() — produces one linked chain of every entry type, including edge cases (float timestamps, None fields, nested JSON).
  Third-party packages (like tau_hub) can self-certify in one test:

from tau_agent.session import verify_session_storage

async def test_my_storage_conforms(tmp_path):
    await verify_session_storage(
        lambda: MyStorage(tmp_path / "db", session_id="one"),
        reopen=lambda: MyStorage(tmp_path / "db", session_id="one"),
    )

Step 3 — Pluggable storage resolution (tau_coding.session_storage)

• create_session_storage(record, factory=None) is the new joint between tau_coding and storage backends.
• Resolution order: explicit factory > TAU_SESSION_STORAGE env var (package.module: attribute) > default JSONL at record.path.
• SessionStorageResolutionError with pointed messages for bad specs, missing modules/attributes, and non-callable factories.
  No changes to existing behaviour when nothing is configured.

Resolves #341

Note:

But still not updated the documentation (first step from #341).