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/search/hybrid.search.service.ts

@@ -101,9 +101,12 @@ export class HybridSearchService {
 	 *
 	 * @param query   - Raw search string (preprocessing handled by SearchService).
 	 * @param options - Search parameters including repositoryId and alpha blend.
-	 * @returns Ranked array of SnippetSearchResult, deduplicated by snippet ID.
+	 * @returns Object with ranked results array and the search mode actually used.
 	 */
-	async search(query: string, options: HybridSearchOptions): Promise<SnippetSearchResult[]> {
+	async search(
+		query: string,
+		options: HybridSearchOptions
+	): Promise<{ results: SnippetSearchResult[]; searchModeUsed: string }> {
 		const limit = options.limit ?? 20;
 		const mode = options.searchMode ?? 'auto';
 
@@ -127,12 +130,12 @@ export class HybridSearchService {
 		// Semantic mode: skip FTS entirely and use vector search only.
 		if (mode === 'semantic') {
 			if (!this.embeddingProvider || !query.trim()) {
-				return [];
+				return { results: [], searchModeUsed: 'semantic' };
 			}
 
 			const embeddings = await this.embeddingProvider.embed([query]);
 			if (embeddings.length === 0) {
-				return [];
+				return { results: [], searchModeUsed: 'semantic' };
 			}
 
 			const queryEmbedding = embeddings[0].values;
@@ -144,7 +147,10 @@ export class HybridSearchService {
 			});
 
 			const topIds = vectorResults.slice(0, limit).map((r) => r.snippetId);
-			return this.fetchSnippetsByIds(topIds, options.repositoryId, options.type);
+			return {
+				results: this.fetchSnippetsByIds(topIds, options.repositoryId, options.type),
+				searchModeUsed: 'semantic'
+			};
 		}
 
 		// FTS5 mode (keyword) or hybrid/auto modes: try FTS first.
@@ -157,7 +163,7 @@ export class HybridSearchService {
 
 		// Degenerate cases: no provider or pure FTS5 mode.
 		if (!this.embeddingProvider || alpha === 0) {
-			return ftsResults.slice(0, limit);
+			return { results: ftsResults.slice(0, limit), searchModeUsed: 'keyword' };
 		}
 
 		// For auto/hybrid modes: if FTS yielded results, use them; otherwise try vector.
@@ -168,14 +174,14 @@ export class HybridSearchService {
 			// No FTS results: try vector search as a fallback in auto/hybrid modes.
 			if (!query.trim()) {
 				// Query is empty; no point embedding it.
-				return [];
+				return { results: [], searchModeUsed: 'keyword_fallback' };
 			}
 
 			const embeddings = await this.embeddingProvider.embed([query]);
 
 			// If provider fails (Noop returns empty array), we're done.
 			if (embeddings.length === 0) {
-				return [];
+				return { results: [], searchModeUsed: 'keyword_fallback' };
 			}
 
 			const queryEmbedding = embeddings[0].values;
@@ -187,7 +193,10 @@ export class HybridSearchService {
 			});
 
 			const topIds = vectorResults.slice(0, limit).map((r) => r.snippetId);
-			return this.fetchSnippetsByIds(topIds, options.repositoryId, options.type);
+			return {
+				results: this.fetchSnippetsByIds(topIds, options.repositoryId, options.type),
+				searchModeUsed: 'keyword_fallback'
+			};
 		}
 
 		// FTS has results: use RRF to blend with vector search (if alpha < 1).
@@ -195,7 +204,7 @@ export class HybridSearchService {
 
 		// Provider may be a Noop (returns empty array) — fall back to FTS gracefully.
 		if (embeddings.length === 0) {
-			return ftsResults.slice(0, limit);
+			return { results: ftsResults.slice(0, limit), searchModeUsed: 'keyword' };
 		}
 
 		const queryEmbedding = embeddings[0].values;
@@ -210,7 +219,10 @@ export class HybridSearchService {
 		// Pure vector mode: skip RRF and return vector results directly.
 		if (alpha === 1) {
 			const topIds = vectorResults.slice(0, limit).map((r) => r.snippetId);
-			return this.fetchSnippetsByIds(topIds, options.repositoryId, options.type);
+			return {
+				results: this.fetchSnippetsByIds(topIds, options.repositoryId, options.type),
+				searchModeUsed: 'semantic'
+			};
 		}
 
 		// Build ranked lists for RRF.  Score field is unused by RRF — only
@@ -221,7 +233,10 @@ export class HybridSearchService {
 		const fused = reciprocalRankFusion(ftsRanked, vecRanked);
 
 		const topIds = fused.slice(0, limit).map((r) => r.id);
-		return this.fetchSnippetsByIds(topIds, options.repositoryId, options.type);
+		return {
+			results: this.fetchSnippetsByIds(topIds, options.repositoryId, options.type),
+			searchModeUsed: 'hybrid'
+		};
 	}
 
 	// -------------------------------------------------------------------------