fix(cli): raise a clear error when conformance replay recordings are missing

[chelsealong]

Fixes #7128

Root cause

adk conformance test talks to an already-running adk web server over
HTTP (AdkWebServerClient, default http://127.0.0.1:8000). It never
starts a server itself, so whichever plugins the server was started with
determine what conformance operations actually work.

• Record mode needs RecordingsPlugin loaded server-side (via
  adk web --extra_plugins=google.adk.cli.plugins.recordings_plugin.RecordingsPlugin)
  to capture LLM/tool interactions into generated-recordings.yaml.
• Replay mode (adk conformance test) needs the different
  ReplayPlugin (google.adk.cli.plugins.replay_plugin.ReplayPlugin) to
  load those recordings into session state (_adk_replay_recordings) so
  _ConformanceTestGemini can mock the model.

ReplayPlugin is never referenced anywhere outside its own module (only
covered by its own unit tests), so there is no built-in or documented way
to discover that it's required. The issue's repro follows the only
plugin hinted at in the workflow — RecordingsPlugin — and reuses the
same running server for both the record and test phases. Because that
server never loaded ReplayPlugin, _ConformanceTestGemini.__init__
read config.get('_adk_replay_recordings') as None and crashed with an
unrelated AttributeError when indexing into it:

Error: Replay verification failed: AttributeError: 'NoneType' object has no attribute 'recordings'

Fix

_ConformanceTestGemini.__init__ now checks for the missing recordings
and raises a ReplayVerificationError that names the actual problem and
how to fix it (start the server with ReplayPlugin loaded), instead of
crashing on an unrelated attribute access a few lines later. This follows
the same "turn a cryptic runtime error into an actionable one" pattern
already used elsewhere in this codebase (e.g. the ImportError raised
for VertexAiSearchTool's GCP bypass).

I did not change the server/plugin wiring itself (e.g. auto-registering
ReplayPlugin/RecordingsPlugin on every adk web server) since that
changes production request-path behavior for all adk web users and is
a larger design call than a minimal bug fix should make unilaterally.

Test plan

Added tests/unittests/cli/conformance/test_conformance_test_google_llm.py:

• test_missing_recordings_raises_actionable_error — constructs
  _ConformanceTestGemini with a config missing _adk_replay_recordings
  (the exact state produced when ReplayPlugin isn't loaded) and asserts
  a ReplayVerificationError mentioning ReplayPlugin is raised.
• test_present_recordings_does_not_raise — sanity check that a
  properly populated config still constructs fine.

Verified the first test reproduces the issue's exact AttributeError
against the pre-fix code (via git checkout HEAD~1 -- <file>, confirmed
failing, then restored the fix):

E     AttributeError: 'NoneType' object has no attribute 'recordings'
src/google/adk/cli/conformance/_conformance_test_google_llm.py:248: AttributeError
1 failed, 1 passed in 0.84s

With the fix applied:

tests/unittests/cli/conformance/test_conformance_test_google_llm.py::test_missing_recordings_raises_actionable_error PASSED
tests/unittests/cli/conformance/test_conformance_test_google_llm.py::test_present_recordings_does_not_raise PASSED
2 passed in 0.82s

Also ran the full tests/unittests/cli suite (includes existing
conformance and replay-plugin tests) to check for regressions:

1048 passed, 5 skipped, 3 xfailed in 50.61s

isort/pyink report no formatting changes needed on the touched files.

AI assistance disclosure

This change was developed with the help of Claude Code (Anthropic).

🤖 Generated with Claude Code