fix(oci): retry transient token requests

[abhinavgautam01]

Summary

Fixes #266.

• Retry OCI Bearer-token requests after transient network and DNS failures.
• Retry rate-limited (429) and server-error (5xx) token responses.
• Use bounded exponential backoff: 500ms, 1s and 2s.
• Respect request cancellation and deadlines while waiting to retry.
• Avoid retrying permanent token failures such as invalid credentials (401).
• Add regression coverage for temporary DNS failures and permanent authentication failures.

Validation

• gofmt
• go tool golangci-lint run ./...
• go vet ./...
• go test -race ./...
• git diff --check

[Copilot]

Pull request overview

This PR improves OCI/Docker registry authentication robustness by adding retry logic around bearer-token acquisition, aiming to mitigate transient network/DNS failures and certain retryable HTTP responses, with accompanying regression tests.

Changes:

• Add bounded exponential backoff retries for token fetch on transient request failures and on 429 / 5xx responses.
• Refactor token response handling into helpers (decodeTokenResponse, tokenResponseError) and add retry decision helpers.
• Add tests covering temporary DNS failures (retry) and permanent auth failures (no retry).

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
internal/httpclient/transport.go	Implements token-request retry loop with backoff, plus helper functions for decoding/errors and retry decisions.
internal/httpclient/transport_test.go	Adds regression tests verifying retries on temporary DNS failures and no retries on 401 responses.

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