fix(resolve): probe .mts/.cts extensions for extension-less imports

[carlos-alm]

Summary

Discovered while fixing #2299 (the .mjs→.mts/.cjs→.cts emitted-extension remap). A separate, distinct code path — resolving an extension-less relative specifier like import './foo' by probing a fixed list of candidate extensions — never tries .mts/.cts in either engine, even though both engines otherwise recognize and parse .mts/.cts files (#2073).

• TS: the extension-probing loop in resolveImportPathJS (src/domain/graph/resolve.ts) tried ['.ts', '.tsx', '.js', '.jsx', '.mjs', '.py', '.pyi', ...index variants] — no .mts/.cts.
• Rust: probe_known_extensions's EXTENSIONS constant (crates/codegraph-core/src/domain/graph/resolve.rs) had the identical list, same gap.

So import './foo' resolving to a source file foo.mts/foo.cts (as opposed to the emitted-specifier form import './foo.mjs', which #2299 covers) fell through to proximity-based confidence instead of an import-aware match.

Fix

Added .mts/.cts to both extension lists, in both engines, placed alongside .ts/.tsx to keep TypeScript-family extensions grouped (matching the existing ordering convention where TS extensions precede JS extensions in the list).

Test plan

• New tests in tests/unit/resolve.test.ts: extension-less specifier resolves to .mts, extension-less specifier resolves to .cts — reusing the exact fixture files (src/esm/util.mts, src/cjs/legacy.cts) Import resolution doesn't remap .mjs/.cjs specifiers to .mts/.cts sources #2299's own remap tests already set up, but exercising the extension-probing loop directly (no extension on the specifier at all) rather than EMIT_EXTENSION_REMAPS
• New tests in crates/codegraph-core/src/domain/graph/resolve.rs mirroring both cases
• Revert-verified on both engines: temporarily removed .mts/.cts from each list and confirmed the corresponding new tests fail with the exact pre-fix output (resolves to the bare specifier, no extension); restored and confirmed green
• cargo test --lib (full suite) — 1096 passed (up from 1094, +2 new)
• cargo clippy --lib -- -D warnings — clean
• npm test (full suite) — 339 files, 5449 passed
• npx tsc --noEmit — clean
• npm run lint — clean

Closes #2464

[github-actions]

Heads up: this PR references #2073 without a closing keyword (Closes #N / Fixes #N). If this PR fully resolves #2073, update the description so the issue auto-closes on merge — otherwise disregard this comment.

[greptile-apps]

Greptile Summary

The PR extends extension-less import resolution to recognize .mts and .cts files in both resolver engines and completes the previously missing directory-index behavior.

• Adds direct .mts and .cts extension probes in the TypeScript and Rust resolvers.
• Adds /index.mts and /index.cts probes in both implementations.
• Adds mirrored tests for direct-file and directory-index resolution.

Confidence Score: 5/5

The PR appears safe to merge.

No blocking failure remains.

Important Files Changed

Filename	Overview
crates/codegraph-core/src/domain/graph/resolve.rs	Adds .mts/.cts direct and index probes to the native resolver with coverage for all four cases.
src/domain/graph/resolve.ts	Keeps the TypeScript resolver in parity by adding matching direct and directory-index probes.
tests/unit/resolve.test.ts	Adds filesystem fixtures and regression tests for extension-less .mts/.cts file and directory imports.

_{Reviews (2): Last reviewed commit: "fix(resolve): also probe /index.mts and ..." | Re-trigger Greptile}

[github-actions]

Codegraph Impact Analysis

1 functions changed → 19 callers affected across 4 files

• resolveImportPathJS in src/domain/graph/resolve.ts:1202 (19 transitive callers)

[carlos-alm]

@greptileai