STO-67: fast-fail Model Repo storage quota check

[jebenexer]

Resolves STO-67.
Server-side counterpart: runpod/RunPod#5929.

#339 (STO-366, pre-signed URL batching) is independent — this check runs before
session creation either way.

Problem

runpodctl already knows the total model size before uploading, but goes
straight into session creation with no check against the account's storage
quota. An over-quota upload should fail immediately with a useful message,
not after creating a model and partial upload sessions server-side.

Changes

• api.GetModelRepoStorageUsage wraps the new modelRepoStorageUsage query.
• checkModelRepoStorageQuota compares the total upload size against
  availableBytes and returns an actionable error when it would be exceeded.
  It fails open: a query error or unparseable response warns and proceeds,
  since a client-side pre-check must never itself break a valid upload. Skipped
  when enforcement is off (Enforced: false) or no quota is set
  (AvailableBytes: nil). createModelRepoUpload's server-side gate is still
  the real limit.
• Wired into runAddModel after file-name/file-size validation and before
  addModelToRepo, covering both --model-path and single-file flows.

Testing

• go vet ./..., go build ./..., go test ./..., govulncheck ./...
• checkModelRepoStorageQuota allow/deny/boundary/fail-open, and
  modelUploadRequestedBytes' directory-vs-flag size resolution.
• API-level tests for GetModelRepoStorageUsage, including its entry in the
  STO-357 typed-error table.
• End-to-end test confirming addModelToRepo and createModelRepoUpload are
  never reached when the check rejects the upload.

---

Note

Low Risk
Adds an optional pre-upload check that fails open on API errors; no change to server enforcement or non-upload flows (e.g. Hugging Face mirror).

Overview
Adds a client-side Model Repo storage quota pre-check so runpodctl model add can reject over-quota uploads before addModelToRepo or upload sessions are created.

The API layer gains ModelRepoStorageUsage and GetModelRepoStorageUsage, calling the modelRepoStorageUsage GraphQL query (optional owner, byte fields as strings). In model add, checkModelRepoStorageQuota compares the planned upload size—from --model-path totals or --file-size—against availableBytes when enforced is true, returning a clear error if the upload would exceed quota. The check fails open on query/parse errors (stderr warning only) so the CLI never blocks uploads when usage cannot be read; server-side enforcement on createModelRepoUpload remains authoritative. Quota validation runs after upload flag validation and before any model creation.

Tests cover API parsing/GraphQL errors, quota allow/deny/boundary/fail-open behavior, byte-size resolution, and an integration case that addModelToRepo / createModelRepoUpload are not invoked when over quota.

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[jebenexer]

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