test: add subprocess e2e suite driving plugins as Flow Launcher does

[Garulf]

Summary

Adds tests/e2e/: end-to-end tests that spawn fixture plugins as real subprocesses using the exact process mechanics Flow Launcher uses, asserting on the raw stdout stream.

• V1: fresh process per request, JSON-RPC request serialized into argv[1], response deserialized from stdout — mirroring JsonRPCPlugin in the host.
• V2: one persistent process, newline-delimited JSON-RPC over stdin/stdout, driven with the host's actual message shapes (params: [{search: ...}, Settings.Inner], lifecycle methods, $/ notifications).

Two self-contained fixture plugins (tests/e2e/fixtures/{v1,v2}_plugin/) with real plugin.json manifests, so launcher auto-detection (Language: python vs python_v2) is exercised end-to-end too.

What this catches that in-process tests can't

The existing integration tests patch sys.stdin/stdout in-process — great protocol coverage, but blind to:

• stdout buffering / flush deadlocks over real pipes
• encoding issues (UTF-8 vs legacy codepages on Windows)
• stray output (logging, print, warnings) corrupting the JSON stream
• process lifecycle: clean exit on close, surviving malformed input, one process serving many queries

Coverage

14 tests: query/context_menu roundtrips, settings delivery (both protocols), unicode, empty query, initialize/close, $/cancelRequest ignored silently, malformed line resilience, stdout purity, process persistence. Subprocesses run with cwd=tmp_path so plugin.log never lands in the repo, and a reader thread with timeouts ensures a hung plugin fails fast instead of hanging CI.

CI

• Runs automatically in the existing matrix (pytest tests picks up tests/e2e/).
• New e2e-windows job on windows-latest (tox -e py311 -- tests/e2e), since Flow Launcher plugins only ever run on Windows.

Verification

tox -e lint,type,py38,py312 all green — 148 tests (134 existing + 14 new).