feat(agent): let custom agents opt into parallel tool execution

[hjxccc]

Description

AgentEngine.executeToolCalls already branches to executeToolCallsParallel when AgentConfig.ParallelToolCalls is set (internal/agent/act.go:230), and that branch is fully implemented — errgroup, results collected in original order, siblings not cancelled on individual failure.

But nothing ever sets the flag. buildAgentConfig copies CustomAgentConfig into the runtime AgentConfig field by field, and this field was missing from the bridge, so the gate always read the zero value:

$ grep -rn "ParallelToolCalls:\|\.ParallelToolCalls =" --include=*.go internal/ | grep -v _test | grep -v chat/
internal/agent/think.go:176:  ParallelToolCalls: &parallelToolCalls,

That single hit is chat.Options.ParallelToolCalls (a *bool on the outbound LLM request) — a different struct. types.AgentConfig.ParallelToolCalls has no assignment anywhere, which makes executeToolCallsParallel unreachable for every custom agent.

This PR adds ParallelToolCalls to CustomAgentConfig and maps it in buildAgentConfig, so the existing engine capability becomes reachable.

Defaults to false. Tools are not required to be side-effect free, and agents that depend on tools observing each other's writes must keep running them sequentially. Turning concurrency on for already-saved agents would be a behaviour change rather than a fix, so opting in is explicit.

Type of Change

• 🐛 Bug fix
• ✨ New feature
• 💥 Breaking change
• 📚 Documentation update
• 🎨 Refactor
• ⚡ Performance improvement
• 🧪 Test
• 🔧 Configuration / Build / CI

Related Issue

None — found while reading the agent execution path.

Testing

Go 1.26.0, CGO_ENABLED=1:

• gofmt -l internal/types/custom_agent.go internal/application/service/session_agent_qa.go internal/application/service/agent_parallel_tool_calls_config_test.go — no output
• go vet ./internal/types/ ./internal/application/service/ — clean
• go test ./internal/application/service/ -run ParallelToolCalls -count=1 -v — 3 cases pass

New coverage mirrors the existing agent_memory_config_test.go, which guards the same class of defect (a field silently absent from buildAgentConfig's field-by-field copy):

• TestAgentConfigCarriesTheParallelToolCallsPreference — opted in / opted out both propagate.
• TestAgentConfigDefaultsParallelToolCallsOff — an agent that never set the field stays sequential.

The focused packages were validated. Full-repository make test was not run; the change is two field additions plus their unit tests.

Checklist

• git diff --check origin/main...HEAD passes
• Changed source files are formatted
• Targeted tests for the changed packages pass
• Diff-scoped lint passes where applicable (go vet for the changed packages)
• Full-repository checks were run, or any unrelated/environment-dependent failures are documented above
• Self-reviewed the code
• Added/updated tests covering the change
• Updated related documentation (not applicable; the field is self-documenting in the config struct and defaults to existing behavior)
• Breaking changes are clearly called out (none — default preserves sequential execution)

Screenshots / Recordings

Not applicable; backend-only change. No frontend toggle is included — the field is settable through the existing agent config API. Happy to add a UI control in the agent editor if maintainers would like it in the same PR.

[hjxccc]

友好 ping 一下 🙏 CI 全绿、无冲突。

一句话：executeToolCallsParallel（act.go）整套已经实现好了，但 types.AgentConfig.ParallelToolCalls 在全仓没有任何赋值点，buildAgentConfig 漏了这个字段，所以这条并行分支对所有自定义智能体都是不可达代码。本 PR 只是把 CustomAgentConfig → AgentConfig 的桥接补齐。

默认 false，存量智能体行为不变——工具不保证无副作用，所以必须显式 opt-in。