bug: incremental indexing can leave stale indexed locations when languages are disabled

## Blocked
This issue is **blocked on PR #135** being merged/adopted: https://github.com/elastic/semantic-code-search-indexer/pull/135

If #135 is not merged, this issue likely needs re-scoping or closing.

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## Problem
Incremental indexing computes `supportedExtensions` from `SEMANTIC_CODE_INDEXER_LANGUAGES`. For modified files, stale indexed locations were previously only removed when the modified file's extension is currently supported.

That means changing `SEMANTIC_CODE_INDEXER_LANGUAGES` (or temporarily disabling a language) can leave stale documents / stale `filePaths` entries in Elasticsearch for files that were previously indexed.

## Suggested fix
- Always call `deleteDocumentsByFilePaths` for any changed/deleted paths (M/D/R-old), regardless of whether the extension is currently supported.
- Continue to only parse/enqueue files for indexing if the extension is supported.

## Test plan
- Unit test: feed a git diff output containing an `M\tfoo.unsupported` and assert it is included in deleteDocumentsByFilePaths args even though it is not enqueued.