fix(testing): let the Phase 0A storage fake take the kwargs the real Blob takes

[abunet]

What changed and why

_LocalBlob in the Phase 0A replay harness accepts only the positional argument on
upload_from_filename, upload_from_string and download_to_filename. The real
google.cloud.storage.Blob it stands in for accepts more — content_type among them — so a caller
that passes one dies inside the harness with a bare TypeError, surfacing three layers away as an
HTTP 503 from the sync router with nothing pointing at the fake:

ERROR routers.sync: event=sync_dispatch outcome=staging_failed lane=fresh exception_type=TypeError
POST /v2/sync-local-files ... 503 Service Unavailable
{"code":"sync_dispatch_unavailable","detail":"Sync dispatch is temporarily unavailable; ..."}
FEASIBILITY OUTCOME: BLOCKED

The file's own comment (line 48) calls itself a "copy of sync_cloud_tasks_stack pattern, not
import". That original — backend/testing/sync_cloud_tasks_stack/storage.py, backing the
blocking gauntlet — already tolerates *_args, **_kwargs on all three methods. The copy dropped
it. This restores it, and nothing else: three signatures, no behaviour change.

Closes #12186.

Product invariants affected

none

How it was verified

The defect is latent on main today — upload_syncing_temporal_file calls the blob with no keyword —
so "revert the fix and watch a test go red" would prove nothing. Verified the other way, by adding a
caller that uses the real API (content_type='application/octet-stream') and running the harness:

worktree state	npm run test:replay-harness-phase0a:emulator
main + that caller, fake unfixed	FEASIBILITY OUTCOME: BLOCKED (base, mutant-unguarded, mutant-guarded)
main + that caller, fake fixed	FEASIBILITY OUTCOME: GREEN/BOUNDED
this branch as it ships (caller reverted, fix only)	FEASIBILITY OUTCOME: GREEN/BOUNDED

So the change is load-bearing for any caller that uses the documented API, and inert for main as it
stands. Ran against the real Firestore emulator and a real redis-server, the same way the CI job
does; the probe caller was reverted before committing and is not part of the diff.

python3 .github/scripts/run_checks.py --base upstream/main --head HEAD --lane ci passes.

Tests

No test change. The fake lives in backend/testing/replay_harness_phase0a/apps.py, which installs the
egress guard and rebinds google.auth at import time — importing it from a unit test would pollute
sys.modules, which tests/unit/test_sys_modules_hermeticity.py exists to prevent. The behavioural
coverage is the harness run itself, shown above. Happy to add a test if you would rather the fake were
extracted into an importable module, but that seemed like more surface than this fix warrants.

Failure class (fixes)

Failure-Class: none

[kodjima33]

Confidence 5/5 (backend/testing): fixes the storage test-fake's method signatures to accept the same kwargs the real GCS Blob takes, matching an established pattern. Trivial, scoped, CI green.