test: Add degenerate-input corpus and MCP tool matrix

[Xuepoo]

Summary

Characterization coverage closing three review test-gaps (tests only, no production changes):

1. ctx-symbol degenerate corpus: BOM-prefixed sources across 6 languages (ranges exclude BOM), CRLF corpus x6, empty/whitespace files across 14 extensions, garbage ERROR-node trees — all no-panic + deterministic.
2. MCP tool matrix: every tool (outline/symbol/read/deps/exec) round-tripped over stdio JSON-RPC in one session plus error mapping (nonexistent file, mistyped arg naming key+type, unknown tool).
3. ctx-exec edge pins: no-trailing-newline passthrough parity, single-line outputs, collapse_threshold boundary +/-1 via stats.collapsed, zero-push stream, stderr-only CLI wiring.

All tests pass on current main; zero characterization failures found.

Closes CTX-0040.

Testing

• 216 passed / 0 failed / 0 ignored; fmt / clippy -D warnings green. MCP test server now isolates XDG_CONFIG_HOME from ambient user config.