fix(tui): make subagent api timeout configurable

[LING71671]

Summary

Closes #1806.

This PR makes the sub-agent per-step model API timeout configurable without changing the legacy default:

• adds [subagents] api_timeout_secs
• keeps the default at 120 seconds for backwards compatibility
• caps configured values at 1800 seconds
• threads the resolved timeout through EngineConfig into SubAgentRuntime
• uses the configured timeout around each sub-agent create_message call
• documents the setting in docs/CONFIGURATION.md and docs/SUBAGENTS.md

Scope

This intentionally implements the minimal timeout-control fix from the issue. It does not change sub-agent tool permissions, write access, streaming, or checkpoint behavior.

Validation

Passed:

• cargo fmt --all -- --check
• git diff --check
• cargo check -p deepseek-tui --bin deepseek-tui
• cargo test -p deepseek-tui --bin deepseek-tui subagent_api_timeout_defaults_to_legacy_120s
• cargo test -p deepseek-tui --bin deepseek-tui subagent_api_timeout_only_clamps_to_maximum
• cargo test -p deepseek-tui --bin deepseek-tui child_runtime_preserves_api_timeout
• cargo test -p deepseek-tui --bin deepseek-tui subagent_runtime_default_api_timeout_is_legacy_120s
• cargo test -p deepseek-tui --bin deepseek-tui tools::subagent::tests::
• cargo build
• cargo clippy --workspace --all-targets --all-features

Notes:

• cargo build / cargo clippy still report pre-existing warnings in schema_migration.rs.
• cargo clippy also still reports the pre-existing needless_return warnings in tui/ui.rs.

[gemini-code-assist]

Code Review

This pull request adds a configurable per-step model API timeout for sub-agents, including configuration updates, runtime propagation, and documentation. The reviewer suggests enforcing a minimum timeout of 1 second to prevent configuration errors and ensuring that all documentation and tests consistently reflect the valid range of 1 to 1800 seconds.

[Hmbown]

Thanks for the configurable subagent API timeout work. This was harvested into v0.8.40 release PR #1823 as commit ad122fd, and #1823 is now CI-green. Closing as superseded by the release branch; thank you for improving long-running subagent behavior.

[LING71671]

Thanks for harvesting this into v0.8.40. Glad the subagent timeout work made it into the release branch.