Add oauth_resource handling for MCP login flows (#12866)

Addresses bug https://github.com/openai/codex/issues/12589

Builds on community PR #12763.

This adds `oauth_resource` support for MCP `streamable_http` servers and
wires it through the relevant config and login paths. It fixes the bug
where the configured OAuth resource was not reliably included in the
authorization request, causing MCP login to omit the expected
`resource` parameter.

codex-rs/core/tests/suite/truncation.rs

@@ -370,6 +370,7 @@ async fn mcp_tool_call_output_exceeds_limit_truncated_for_model() -> Result<()>
                 enabled_tools: None,
                 disabled_tools: None,
                 scopes: None,
+                oauth_resource: None,
             },
         );
         config
@@ -464,6 +465,7 @@ async fn mcp_image_output_preserves_image_and_no_text_summary() -> Result<()> {
                 enabled_tools: None,
                 disabled_tools: None,
                 scopes: None,
+                oauth_resource: None,
             },
         );
         config
@@ -729,6 +731,7 @@ async fn mcp_tool_call_output_not_truncated_with_custom_limit() -> Result<()> {
                 enabled_tools: None,
                 disabled_tools: None,
                 scopes: None,
+                oauth_resource: None,
             },
         );
         config