fix(MULTIVERSION-0001): fix version isolation, 404 on unknown version, commit-hash lookup, and searchModeUsed

Bug 1: Thread version tag from run() into crawl() via getVersionTag() helper so
LocalCrawler and GithubCrawler receive the correct ref when indexing a named
version instead of always crawling HEAD.

Bug 2: Return HTTP 404 with code VERSION_NOT_FOUND when a requested version tag
is not found in repository_versions, instead of silently falling back to a
cross-version mixed result set.

Bug 4: Before returning 404, attempt a commit_hash prefix match (min 7 chars)
so callers can request a version by full or short SHA.

Bug 3: Change HybridSearchService.search() to return
{ results, searchModeUsed } and propagate searchModeUsed through
ContextResponseMetadata and ContextJsonResponseDto so callers can see which
strategy (keyword / semantic / hybrid / keyword_fallback) was actually used.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/lib/server/pipeline/indexing.pipeline.test.ts

@@ -729,4 +729,46 @@ describe('IndexingPipeline', () => {
 		// State should remain 'pending' — pipeline with no versionId must not touch it
 		expect(version.state).toBe('pending');
 	});
+
+	it('calls LocalCrawler with ref=v1.2.0 when job has a versionId with tag v1.2.0', async () => {
+		const versionId = insertVersion(db, { tag: 'v1.2.0', state: 'pending' });
+
+		const crawl = vi.fn().mockResolvedValue({
+			files: [],
+			totalFiles: 0,
+			skippedFiles: 0,
+			branch: 'main',
+			commitSha: 'abc'
+		});
+
+		const pipeline = new IndexingPipeline(db, vi.fn() as never, { crawl } as never, null);
+		const job = makeJob('/test/repo', versionId);
+
+		await pipeline.run(job as never);
+
+		expect(crawl).toHaveBeenCalledWith({
+			rootPath: '/tmp/test-repo',
+			ref: 'v1.2.0'
+		});
+	});
+
+	it('calls LocalCrawler with ref=undefined when job has no versionId (main-branch)', async () => {
+		const crawl = vi.fn().mockResolvedValue({
+			files: [],
+			totalFiles: 0,
+			skippedFiles: 0,
+			branch: 'main',
+			commitSha: 'abc'
+		});
+
+		const pipeline = new IndexingPipeline(db, vi.fn() as never, { crawl } as never, null);
+		const job = makeJob('/test/repo'); // no versionId
+
+		await pipeline.run(job as never);
+
+		expect(crawl).toHaveBeenCalledWith({
+			rootPath: '/tmp/test-repo',
+			ref: undefined
+		});
+	});
 });