Add update_config write path (TWS 10.44 / server 221)

[wboayue]

Summary

Implements Option 2 of the config feature — the update_config write path (IBKR added it in TWS 10.44). Adds a fluent Client::update_config() (sync + async) that pushes partial edits to the running gateway's configuration and returns an UpdateConfigResponse reporting changed fields, errors, and warnings.

Follows PR #707 (the read path); reuses the read-path domain types as the write-side input.

API shape

Fluent builder (chosen over a struct-input form), matching the house style:

let response = client
    .update_config()
    .api(ApiConfig {
        settings: Some(ApiSettings { socket_port: Some(7497), ..Default::default() }),
        ..Default::default()
    })
    .message(MessageSetting { id: Some(131), enabled: Some(false), ..Default::default() })
    .accept_warning(warning)      // echo a warning from a prior response
    .reset_api_order_sequence()
    .submit()                     // sync
    .await?;                      // async

Semantics

This is a one-shot request → one-shot response — there is no library-level re-submit loop (confirmed against C# EClient.updateConfig and the Python ConfigSamples). If the response's warnings are non-empty the edit was not applied; the caller re-submits the same edit with each warning echoed via .accept_warning(...) to proceed. That acknowledgement round-trip is application-level, so the library stays a thin one-shot.

What's included

• Fluent UpdateConfigBuilder<'a, C> (src/config/builder/): generic over the client type, feature-agnostic setters, per-feature submit() in sync_impl.rs/async_impl.rs — the established dual-feature builder pattern (rule 12, mirrors OrderBuilder).
• Reuses the read-path domain types (ApiConfig, ApiSettings, OrdersConfig, LockAndExit, MessageSetting) as write input — all Option fields with Default, so partial edits are ..Default::default().
• New domain types: UpdateConfigResponse { status, message, changed_fields, errors, warnings } and ConfigWarning.
• Domain→proto encoders (config/common/encoders.rs) mirror the read path's proto→domain decoders; decoder for UpdateConfigResponse.
• Features::UPDATE_CONFIG (server 221); UpdateConfigResponse registered in text_request_id_field so it routes by request_id (rule 17).
• Field-minimal testdata builder; sync + async examples registered in Cargo.toml; CHANGELOG.md entry.

Tests drive the real client API and verify the captured wire request by decoding proto::UpdateConfigRequest from the sent bytes (rule 10), plus response decode, server-version rejection, and empty-stream paths.

Verification

• cargo fmt; clippy ×3 configs (-D warnings); rustdoc ×3 configs (-D warnings); sync + async doc-tests.
• Full lib tests green: default (1319), sync-only (1335), all-features (1663); both integration crates build.
• 100% line coverage on every new config file.
• Live smoke test deferred: unlike the read path, update_config mutates gateway settings, so it was not run against the live paper gateway.