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/models/context-response.ts

@@ -173,6 +173,7 @@ export class ContextJsonResponseDto {
 	repository: ContextRepositoryJsonDto | null;
 	version: ContextVersionJsonDto | null;
 	resultCount: number;
+	searchModeUsed: string;
 
 	constructor(props: ContextJsonResponseDto) {
 		this.snippets = props.snippets;
@@ -182,5 +183,6 @@ export class ContextJsonResponseDto {
 		this.repository = props.repository;
 		this.version = props.version;
 		this.resultCount = props.resultCount;
+		this.searchModeUsed = props.searchModeUsed;
 	}
 }