feat(distributed): resumable file uploads via HTTP Content-Range

[localai-bot]

Summary

Large model GGUFs (multi-GB) transferred between master and worker over flaky / bandwidth-throttled paths (e.g. libp2p relays with byte caps) used to restart from byte 0 on every transport error. This change adds standard HTTP Range/resume semantics to the worker's `PUT /v1/files/` endpoint and teaches the master-side `HTTPFileStager` to consult the worker for the last accepted offset and resume from there.

Server side — `file_transfer_server.go`

• PUT honors `Content-Range: bytes -/`. The handler validates that `` matches the current on-disk size; mismatches return 416 with the actual size in `X-File-Size`.
• Mid-upload chunks return 308 Permanent Redirect ("Resume Incomplete") with the new size, so the client can keep going.
• Optional `X-Content-SHA256` request header binds an upload to a target hash; cross-attempt drift returns 409. On the final chunk the server re-computes SHA-256 and returns 400 if it doesn't match.
• HEAD advertises `Accept-Ranges: bytes` + `Content-Length` and exposes `X-Target-SHA256` for in-progress files (so clients can resume only when the partial bytes belong to the file they want to upload).
• Legacy PUTs with no `Content-Range` keep the original truncate-create semantics — zero behavior change on the happy path.

Client side — `file_stager_http.go`

• Pre-PUT HEAD probe reads `X-File-Size` + `X-Target-SHA256` to determine the resume offset.
• `doUpload` seeks to that offset and sends `Content-Range` + `X-Content-SHA256`.
• Retry loop switches from fixed 3 attempts / 5s-10s-20s backoff to an outer time budget (default 1h, override via `LOCALAI_FILE_TRANSFER_BUDGET`) with exponential backoff (1s → 30s cap), so a 5GB upload over a flaky link can outlast many short disconnects.
• 308 and 416 responses are treated as transient: the next iteration re-HEADs to learn the correct offset.
• The retry-budget context is derived from, not replacing, the caller's ctx — so an upstream routing-model timeout/cancellation still propagates and aborts the loop immediately.

Tests

8 new specs in `file_transfer_server_test.go`:

• two-chunk `Content-Range` round-trip produces correct file + sidecar
• 416 on offset mismatch
• 409 on SHA drift between chunks
• 400 on final-hash mismatch
• HEAD on partial upload exposes `X-Target-SHA256` (not a misleading hash-of-partial-bytes)
• pre-existing finished file with a different hash transparently overwritten on a new PUT starting at byte 0
• end-to-end resume via `EnsureRemote` against a worker that already holds a partial file
• mid-stream connection drop on attempt llama-cli: command not found #1 recovered by attempt windows compatibility? #2

Full nodes suite (267 specs, 147s) green.