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/routes/api/v1/api-contract.integration.test.ts

@@ -486,4 +486,83 @@ describe('API contract integration', () => {
 			isLocal: false
 		});
 	});
+
+	it('GET /api/v1/context returns 404 with VERSION_NOT_FOUND when version does not exist', async () => {
+		const repositoryId = seedRepo(db);
+
+		const response = await getContext({
+			url: new URL(
+				`http://test/api/v1/context?libraryId=${encodeURIComponent(`${repositoryId}/v99.0.0`)}&query=${encodeURIComponent('foo')}`
+			)
+		} as never);
+
+		expect(response.status).toBe(404);
+		const body = await response.json();
+		expect(body.code).toBe('VERSION_NOT_FOUND');
+	});
+
+	it('GET /api/v1/context resolves a version by full commit SHA', async () => {
+		const repositoryId = seedRepo(db);
+		const fullSha = 'a'.repeat(40);
+
+		// Insert version with a commit_hash
+		db.prepare(
+			`INSERT INTO repository_versions
+			 (id, repository_id, tag, commit_hash, state, total_snippets, indexed_at, created_at)
+			 VALUES (?, ?, ?, ?, 'indexed', 0, ?, ?)`
+		).run(`${repositoryId}/v2.0.0`, repositoryId, 'v2.0.0', fullSha, NOW_S, NOW_S);
+
+		const response = await getContext({
+			url: new URL(
+				`http://test/api/v1/context?libraryId=${encodeURIComponent(`${repositoryId}/${fullSha}`)}&query=${encodeURIComponent('anything')}`
+			)
+		} as never);
+
+		expect(response.status).toBe(200);
+		const body = await response.json();
+		expect(body.version?.resolved).toBe('v2.0.0');
+	});
+
+	it('GET /api/v1/context resolves a version by short SHA prefix (8 chars)', async () => {
+		const repositoryId = seedRepo(db);
+		const fullSha = 'b1c2d3e4f5a6b7c8d9e0f1a2b3c4d5e6f7a8b9c0';
+		const shortSha = fullSha.slice(0, 8);
+
+		db.prepare(
+			`INSERT INTO repository_versions
+			 (id, repository_id, tag, commit_hash, state, total_snippets, indexed_at, created_at)
+			 VALUES (?, ?, ?, ?, 'indexed', 0, ?, ?)`
+		).run(`${repositoryId}/v3.0.0`, repositoryId, 'v3.0.0', fullSha, NOW_S, NOW_S);
+
+		const response = await getContext({
+			url: new URL(
+				`http://test/api/v1/context?libraryId=${encodeURIComponent(`${repositoryId}/${shortSha}`)}&query=${encodeURIComponent('anything')}`
+			)
+		} as never);
+
+		expect(response.status).toBe(200);
+		const body = await response.json();
+		expect(body.version?.resolved).toBe('v3.0.0');
+	});
+
+	it('GET /api/v1/context includes searchModeUsed in JSON response', async () => {
+		const repositoryId = seedRepo(db);
+		const documentId = seedDocument(db, repositoryId);
+		seedSnippet(db, {
+			documentId,
+			repositoryId,
+			content: 'search mode used test snippet'
+		});
+
+		const response = await getContext({
+			url: new URL(
+				`http://test/api/v1/context?libraryId=${encodeURIComponent(repositoryId)}&query=${encodeURIComponent('search mode used')}`
+			)
+		} as never);
+
+		expect(response.status).toBe(200);
+		const body = await response.json();
+		expect(body.searchModeUsed).toBeDefined();
+		expect(['keyword', 'semantic', 'hybrid', 'keyword_fallback']).toContain(body.searchModeUsed);
+	});
 });