fix: Uploaded input_file payloads are persisted as full base64 blobs in session rows

[sam-saffron-jarvis]

What changed

• Strip PartFile.FileData.Base64 from persisted session parts when the file has already been saved under term-llm's uploads directory, while retaining the file path, media type, filename, and size metadata.
• Rehydrate path-only uploaded files for Responses native input_file payloads just before request construction, only for first-party upload paths and within the existing upload size cap/policy.
• Added coverage for parsed input_file data URLs, storage sanitization, and native Responses rehydration from a saved upload path.

Why this is high-value

Large web/API file uploads were being stored twice: once as decoded bytes in the uploads directory and again as a ~33% larger base64 string in the SQLite messages.parts JSON. For near-limit files, that bloats session rows by tens of MiB and slows session writes, reads, backups, and resume/search I/O. The fix removes that duplicate serialized payload while preserving native file resend behavior by reading the already-saved upload only when a Responses request can use it.

Validation

• go test ./internal/session ./internal/llm ./cmd
• go build ./...
• go test ./...
• git diff --check