feat: support OkHttp application interceptors

[sylvesterkaczmarek]

Summary

Add application-interceptor support to the SDK's public OkHttp transport builder so users can customise requests and responses without copying the SDK's HttpClient implementation.

Fixes #339.

Problem

The SDK currently exposes several OkHttp transport settings, but there is no supported way to register an OkHttp Interceptor. The documented workaround is to copy the SDK's OkHttp transport implementation and maintain a local fork just to add interceptors.

That is unnecessarily heavy for common use cases such as request tagging, custom observability, organisation-specific headers, response inspection, and integration with existing OkHttp middleware.

Fix

Add a Java-callable builder method:

OkHttpClient.builder().addInterceptor(interceptor)

Configured interceptors are registered as standard OkHttp application interceptors when the underlying transport is built. Multiple calls preserve registration order, matching OkHttp's normal semantics.

The existing public construction path can then use the customised transport directly through ClientOptions and OpenAIClientImpl, without copying SDK source.

Regression coverage

Extended OkHttpClientTest with a WireMock-backed test that:

• installs an application interceptor;
• has the interceptor add a request header;
• performs a real SDK transport request;
• verifies WireMock received the interceptor-added header.

Validation

• branch is based on current upstream main at 81134d727946fb84cc48db98e6c8c53c8a8d22ab;
• branch is 0 commits behind upstream;
• production diff is 7 additions in OkHttpClient.kt;
• existing proxy, timeout, TLS, connection-pool, cancellation, and retry behaviour is unchanged.

Full repository validation is left to GitHub Actions.

Risk

Low. No interceptor is installed by default, so existing clients behave exactly as before. The new behaviour is entirely opt-in and delegates interceptor semantics to OkHttp.

[chatgpt-codex-connector]

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[sylvesterkaczmarek]

I resolved this PR against current main while preserving the intended fix and newer upstream behavior. The branch is now mergeable and no longer has a base conflict. Could a maintainer please re-review it when convenient?

@codex review