fix: rename is_azure_responses_wire_base_url to is_azure_responses_provider (#17965)

## Why

While reviewing https://github.com/openai/codex/pull/17958, the helper
name `is_azure_responses_wire_base_url` looked misleading because the
helper returns true for either the `azure` provider name or an Azure
Responses `base_url`. The new name makes both inputs part of the
contract.

## What

- Rename `is_azure_responses_wire_base_url` to
`is_azure_responses_provider`.
- Move the `openai.azure.` marker into
`matches_azure_responses_base_url` so all base URL marker matching is
centralized.
- Keep `Provider::is_azure_responses_endpoint()` behavior unchanged.

## Verification

- Compared the parent and current implementations.
`name.eq_ignore_ascii_case("azure")` still returns true before
consulting `base_url`, `None` still returns false, base URLs are still
lowercased before marker matching, and the same Azure marker set is
checked.
- Ran `cargo test -p codex-api`.

codex-rs/codex-api/src/lib.rs

@@ -57,7 +57,7 @@ pub use crate::error::ApiError;
 pub use crate::files::upload_local_file;
 pub use crate::provider::Provider;
 pub use crate::provider::RetryConfig;
-pub use crate::provider::is_azure_responses_wire_base_url;
+pub use crate::provider::is_azure_responses_provider;
 pub use crate::requests::Compression;
 pub use crate::sse::stream_from_fixture;
 pub use crate::telemetry::SseTelemetry;