feat(cli): seed adk web sessions from initial_state.json

[BichengWang]

Closes #4961

Problem

adk run --replay can preload session state from a JSON file, but adk web has no equivalent. Anyone iterating on an agent that depends on session state has to re-enter the same values in the UI for every new session.

Change

If <agents_dir>/<app_name>/initial_state.json exists, adk web merges it into the state of each session created through the session endpoints.

• State sent by the client wins, key by key: {**file_state, **client_state}.
• The file is read on each session creation, so edits apply without restarting the server.
• A missing file changes nothing. An unreadable, malformed or non-object file is ignored with a warning.
• The change lives in DevServer._create_session. ApiServer (adk api_server) is untouched.
• The path goes through the existing _get_agent_dir validation.

Not covered: sessions the runner creates itself under --auto_create_session are not seeded, since they never go through _create_session. app: and user: keys in the file follow the normal state scoping, so they write to shared state.

Testing plan

Unit tests in tests/unittests/cli/test_fast_api.py:

• the file seeds a new session
• client state overrides file values, and other file keys are kept
• no file: behavior unchanged
• invalid JSON, a list and a string are ignored
• ApiServer ignores the file

The two seeding tests fail without the change. pytest tests/unittests/cli/test_fast_api.py gives 231 passed, 5 skipped, 1 xfailed. pyink and isort are clean.