add spot pod support to pod create

[vavo]

E-236: add spot pod support to pod create

• resolves feat(pod): add spot pod support with --bid-per-gpu flag #236 by adding --bid-per-gpu to runpodctl pod create.
• maintains full legacy command compatibility (stderr warnings only, stdout unchanged).
• keeps runpodctl as the primary binary; drop-in upgrade for existing users.
• routes gpu pod creation with --bid-per-gpu through the podRentInterruptable graphql mutation instead of the on-demand podFindAndDeployOnDemand path.
• validates that --bid-per-gpu is only used for gpu pods and updates generated pod create docs.
• adds unit coverage for the new flag, cpu validation, and graphql serialization of bidPerGpu.

Test plan:

• go test ./...
• go test -tags e2e ./e2e
• live spot pod create and delete
  attempted with a real api key across multiple gpu ids/bids (RTX 3090, RTX 4090, RTX 4000 Ada, RTX A4000, RTX 5090). the cli hit the new spot mutation path successfully, but every attempt returned market capacity errors: There are no instances available with the request specifications. Please try again later.

[promptless]

Promptless prepared a documentation update related to this change.

Triggered by PR #267

Added the --bid-per-gpu flag to the runpodctl pod create command reference with a usage example and cross-reference to existing spot instances pricing documentation.

Review at https://app.gopromptless.ai/suggestions/c20ab269-4f87-4fd0-b70d-0efd14920e5f