feat(moonshot): add files extraction client

[chen-ran]

Summary

• add a streamed Moonshot Files API client under provider/moonshot/files
• support upload (including a purpose=file-extract convenience method), metadata retrieval, extracted-text reads, and remote deletion
• validate multipart metadata, cap the convenience text reader, and cover request paths, auth, multipart encoding, errors, and input validation
• document how the transport fits Kimi's official file-based QA flow

Why

Kimi's documented PDF/document QA flow is not a native Chat Completions file content part. It requires the application to upload a file to /v1/files, read /v1/files/{id}/content, inject the extracted text into model context, and eventually delete the remote file.

Twilight already maps generic chat content and Kimi-compatible tool schemas, but it had no client for this separate stateful Files API flow. That left applications to hand-roll the provider transport.

Official guide: https://platform.kimi.com/docs/guide/use-kimi-api-for-file-based-qa

Boundaries and impact

• this PR does not automatically upload FilePart values or mutate the Kimi Chat Completions adapter
• application orchestration still owns content-hash caching, token/context budgeting, prompt trust boundaries, retry policy, and remote-file cleanup timing
• existing provider message mapping and non-Moonshot behavior are unchanged
• a follow-up Memoh change can consume this client and implement the application-level policy

Validation

• go build ./...
• go vet ./...
• go test -short -count=1 ./...
• go test -count=1 ./provider/moonshot/files
• golangci-lint run --allow-parallel-runners ./... (0 issues; another repository held golangci-lint's global runner lock)

No live Moonshot upload was run with a real API key.

⚠️ No human QA — this PR has not been verified by a human yet. Remove this line once a human confirms the happy path.