fix(oci): route authorizer token fetches through provided transport

[glours]

What I did
The transport passed to NewResolver was applied only via docker.WithClient. Pass it via docker.WithAuthClient too so the authorizer's OAuth token fetches use it instead of http.DefaultClient.

Related issue
https://docker.atlassian.net/browse/DDB-498

(not mandatory) A picture of a cute animal, if possible in relation to what you did

[Copilot]

Pull request overview

This PR ensures that when a custom HTTP transport is provided to internal/oci.NewResolver, it is used not only for registry traffic but also for the Docker authorizer’s OAuth token fetches (avoiding unintended use of http.DefaultClient).

Changes:

• Pass the provided transport via docker.WithAuthClient so authorizer token requests use the same transport.
• Refactor authorizer option construction for reuse.
• Add a unit test asserting the token endpoint fetch goes through the provided transport.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
internal/oci/resolver.go	Wires the provided transport into both registry client and authorizer token-fetch client.
internal/oci/resolver_test.go	Extends the transport-recording test helper and adds coverage for authorizer token fetch transport usage.

[docker-agent]

Assessment: 🟢 APPROVE

The production fix is correct and well-scoped: extracting authorizer options into authOpts and conditionally appending docker.WithAuthClient(&http.Client{Transport: transport}) ensures OAuth token fetches use the caller-supplied transport rather than http.DefaultClient. Sharing the same http.RoundTripper between two http.Client instances is safe — http.Transport is designed to be shared. The new test correctly exercises the auth dance through the recording transport. Two minor test style observations are left as inline comments.

[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 4 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
internal/oci/resolver.go	66.66%	2 Missing and 2 partials ⚠️

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