Follow-up fixes for Graphify-Labs/graphify#3075

[andrebrait]

This PR carries a set of review-driven fixes to the .graphifyrc language-override feature, found while we carried Graphify-Labs#3075 on our fork and exercised it further. Merging this PR into your branch will update upstream Graphify-Labs#3075 directly with these fixes included.

Rebased on your branch tip 1ad08a9, with no lockfile changes.

• fix(rcfile): validate explicit extensions and cover the override contract — rejects malformed/unsupported extensions declared in .graphifyrc and adds test coverage for the override contract that was previously unverified.
• test: scope hidden-file assertion to the scan root — narrows a test assertion about hidden files to the scan root so it doesn't false-positive on unrelated hidden files elsewhere in the tree.
• fix(config): honor declared languages in extraction and imports — makes extraction and import resolution actually respect the languages declared via overrides, instead of falling back to auto-detection in several extractors (C#, OCaml, Robot Framework, symbol resolution).
• fix(config): refresh graphs when language declarations change — ensures the watch/detect pipeline invalidates and rebuilds graphs when a file's declared language changes, rather than leaving stale graph state in place.
• fix(config): close reviewed override eligibility gaps — closes remaining eligibility gaps found during review across C# dispatch, extraction, and resolution/watch paths so overrides are honored consistently.

Related: Graphify-Labs#3075