Extract agent identity public API request from BaseClient

[vorporeal]

Description

• Extract AuthClientImpl::list_agent_identities() from the remaining endpoint-shaped BaseClient shim into an authenticated public GET helper owned by warp_server_client.
• Keep ambient request headers and failed-response/IAP observation app-owned through small transport hooks, and keep typed HTTP status causes shared so existing retry/error classification remains stable.
• This is intentionally a small stacked cleanup on Extract AuthClient and associated logic to the warp_server_client crate. #11977; it does not migrate unrelated AI or harness endpoints.

Testing

• ./script/format
• cargo check --manifest-path /Users/david/src/warp-auth-public-api-helper/Cargo.toml -p warp_server_client -p warp
• cargo nextest run --manifest-path /Users/david/src/warp-auth-public-api-helper/Cargo.toml -p warp_server_client
• cargo check --manifest-path /Users/david/src/warp-auth-public-api-helper/Cargo.toml --target wasm32-unknown-unknown -p warp_server_client
• cargo clippy --manifest-path /Users/david/src/warp-auth-public-api-helper/Cargo.toml -p warp_server_client -p warp --all-targets --tests -- -D warnings
• Attempted cargo clippy --manifest-path /Users/david/src/warp-auth-public-api-helper/Cargo.toml --workspace --all-targets --all-features --tests -- -D warnings; it is blocked in this environment because command-signatures-v2 requires Node (env: node: No such file or directory).

Agent Mode

• Warp Agent Mode - This PR was created via Warp's AI Agent Mode

CHANGELOG-NONE

Co-Authored-By: Oz oz-agent@warp.dev

Conversation: https://staging.warp.dev/conversation/5eda2cd5-6a44-41a7-9810-2c73b800c0ee
Run: https://oz.staging.warp.dev/runs/019e8064-0bac-7a79-bc93-58e35969a5b4
This PR was generated with Oz.