🤖 fix: handle template imports that outlast the request deadline

[ThomasK33]

Problem

After #116, creating a CoderTemplate with spec.files returns only after Coder finishes importing the template. The server-side limit is CODER_K8S_TEMPLATE_VERSION_BUILD_WAIT_TIMEOUT (default 25 minutes), capped by the request deadline. Requests reach the aggregated API server through kube-apiserver, whose default --request-timeout is 60 seconds, and clients can set shorter deadlines.

If an import takes longer than the effective deadline:

• the client receives a timeout;
• the server creates no template, so nothing half-created is exposed;
• the uploaded file and template version remain in Coder;
• a retry uploads and imports again and can leave additional versions.

The maintained E2E imports a trivial template in seconds, so this path has not been exercised on a real backend. It has not been verified whether kube-apiserver's timeout applies to proxied aggregated requests in every configuration.

Options to evaluate

1. Keep the synchronous Create and document the limit (current state; documented in the aggregated API server how-to).
2. Return early for slow imports and expose import progress through status or conditions, so clients can wait instead of retrying.
3. Make retries idempotent for identical source (for example, reuse a pending or succeeded version with the same content hash).

Trigger and owner

Owner: maintainer desk. Revisit when a user reports a slow-import timeout, when a supported quickstart path is restored, or before advertising large templates as supported.

Follow-up from #105 and #116.

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