fix(extraction): warn when parse errors leave no symbols (#1522)

[colbymchenry]

C++ raw strings with a 16-character delimiter can collapse the whole-file parse and silently leave only a file node. Emit a parse_error warning only when the tree has errors and extraction yields zero non-file symbols, and display that warning in the CLI while keeping indexing successful.

Related to #1522 (Half A). Grammar recovery remains a follow-up: the vendored grammar is unchanged, so the 16-character fixture still has no function symbols. Healthy empty/include-only files and partial parses with surviving symbols stay quiet.

Validation on Linux with Node 22.23.2:

• Confirmed the new extraction and CLI regression tests fail before the fix because the warning is missing.
• npx vitest run __tests__/cpp-raw-string-delimiter-haserror.test.ts — 7 passed.
• npm run build — passed, including viewer and grammar asset checks.
• Ran the focused suite, cli-parse-warning.test.ts, extraction.test.ts, and cli-index-explicit-path.test.ts together — 650 passed.
• Re-initialized both supplied repro directories with the built CLI and queried after_the_raw_string:

Fixture	Index result	Query result
/tmp/cg-1522-repro/d16	1 node, 0 edges; collapse warning visible; exit 0	Function still absent; grammar recovery is deferred
/tmp/cg-1522-repro/d15	2 nodes, 1 edge; no collapse warning; exit 0	Function found