fix(buzz-acp): accept id-keyed config options when resolving model switch

[chillerno1]

Summary

Fixes #2794. Related: #2692.

resolve_model_switch_method() reads the configId key from each session/new configOptions entry and skips entries that lack it. claude-agent-acp (v0.61.0) keys its entries with id, so every model-category entry was skipped, the desired model never matched, and Claude Code sessions fell back to the CLI default from the user's ~/.claude/settings.json. The only trace was a pool::model WARN that never reaches the per-agent log files.

This is the ACP-side half of the symptom reported in #2692. The open desktop-side PRs (#2695, #2701, #2696) inject ANTHROPIC_MODEL at spawn, which masks the problem for spawn-time selection but leaves the config-option switch path broken.

Changes

• resolve_model_switch_method() accepts either configId or id when extracting the config id. The set request is unchanged: the ACP SDK schema takes configId as the request param and the adapter resolves it against its id-keyed entries, so only the read side needed fixing.
• Regression test with an id-keyed configOptions payload mirroring the real adapter response (including models: null, so the unstable fallback path cannot rescue the match).
• Doc comment on extract_model_config_options() notes the key drift.

Testing

cargo test -p buzz-acp --lib: 599 passed, 0 failed. The new test fails on main and passes with this change.

Verified against the real adapter: a stdio JSON-RPC probe of the bundled claude-agent-acp 0.61.0 confirms session/new returns id-keyed config options with opus[1m] present as a value, and the SDK's SetSessionConfigOptionRequest schema accepts {sessionId, configId, value} as sent by session_set_config_option().

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