Fix mypy type errors unmasked in the test suite - #105

Summary

Closes #97.

Now that the duplicate-module error from #82 is fixed (#96 added tests/__init__.py), mypy vesper tests no longer aborts early — which unmasked 61 pre-existing type errors in the test suite (51 at issue-file time, plus new ones added since). This PR makes mypy vesper tests exit 0 while keeping production code clean.

Approach

The dominant pattern was test doubles not satisfying real type signatures. Mirroring the existing Resolver Protocol, this introduces structural Protocols so test doubles satisfy the real types rather than being papered over with ignores:

• RpcClient / RpcSession Protocols (vesper/rpc.py): type the CiderRpcClient session param and all production rpc parameters against them (service, catalog, playback_controller, search_controller, session).
• Resolver stubs inherit FallbackResolver (implements all 5 Protocol methods) so they satisfy the Resolver Protocol structurally.
• RPC stubs gain the missing Protocol methods (set_failure_callback, playback_get, search_library, run_amapi_v3) and default parameter values.
• Mechanical narrowing fixes for dict | None indexing, .text on content unions, utcoffset() None, and a generator fixture return annotation.
• cast() at 7 sites that access stub-only attributes on typed service internals (is_playing, current_track, plan_calls).
• types-jsonschema added to dev deps for the jsonschema import-untyped error.
• _build_session_query_pool now accepts str | SessionSearchSource, matching the existing runtime isinstance(source, str) handling.

Test commands run

.venv/bin/python -m mypy vesper tests   # Success: no issues found in 52 source files
.venv/bin/python -m mypy vesper         # Success: no issues found in 37 source files
.venv/bin/python -m pytest -q           # 270 passed