fix: true parallel tool execution via subscribeOn(Schedulers.io()) in concatMapEager

[YuqiGuo105]

Summary

Fixes #1152 — ToolExecutionMode.PARALLEL was silently executing tools serially. Adding .subscribeOn(Schedulers.io()) inside concatMapEager dispatches each tool's full pipeline to an IO thread, enabling true concurrent execution.

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Root Cause & Fix

Without subscribeOn, all subscriptions run on the calling thread. Blocking tools hold that thread, preventing the next subscription from starting.

// Before (broken) — all subscriptions on calling thread
Observable.fromIterable(validFunctionCalls)
    .concatMapEager(call -> functionCallMapper.apply(call).toObservable());

// After (fixed) — each tool dispatched to an IO thread
Observable.fromIterable(validFunctionCalls)
    .concatMapEager(call ->
        functionCallMapper.apply(call).toObservable().subscribeOn(Schedulers.io()));

Applied identically in both handleFunctionCalls and handleFunctionCallsLive.

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Execution Workflow

Before — serial on calling thread

Time ──────────────────────────────────────────────────────────────►

Calling thread:
  [=== tool_1 (1000 ms) ===][=== tool_2 (1000 ms) ===][=== tool_3 (1000 ms) ===]
                                                                           Total: ~3000 ms

After — concurrent on IO thread pool

Time ──────────────────────────────────────────────────────────────►

Calling thread: [dispatch][dispatch][dispatch][wait ...]
IO thread-1:    [======== tool_1 (1000 ms) ========]
IO thread-2:    [======== tool_2 (1000 ms) ========]
IO thread-3:    [======== tool_3 (1000 ms) ========]
                                                    ▲
                                            Total: ~1000 ms

concatMapEager guarantees results are emitted in input order despite concurrent execution.

---

Tests

All 26 tests pass (Tests run: 26, Failures: 0, Errors: 0). 12 new tests added:

#	Test method	Tool	Mode	Assertion
1	handleFunctionCalls_parallelMode_runsToolsConcurrently	SlowTool ×2 @ 500 ms	PARALLEL	duration < 900 ms
2	handleFunctionCalls_parallelMode_preservesInputOrder	SlowTool slow(500ms), fast(100ms)	PARALLEL	response[0] == slow tool
3	handleFunctionCalls_noneModeDefaultsToParallel_runsToolsConcurrently	SlowTool ×2 @ 500 ms	NONE	duration < 900 ms
4	handleFunctionCalls_sequentialMode_runsToolsSerially	SlowTool ×2 @ 300 ms	SEQUENTIAL	duration ≥ 600 ms
5	handleFunctionCallsLive_parallelMode_runsToolsConcurrently	SlowTool ×2 @ 500 ms	PARALLEL (Live path)	duration < 900 ms
6	handleFunctionCalls_parallelMode_threeTools_allStartConcurrently	SlowTool ×3 @ 500 ms	PARALLEL	start times within 150 ms spread
7	handleFunctionCalls_blockingTools_parallel_executesInParallel	BlockingTool ×3 @ 1000 ms	PARALLEL	duration < 1500 ms
8	handleFunctionCalls_blockingTools_defaultNoneMode_executesInParallel	BlockingTool ×2 @ 1000 ms	NONE	duration < 1500 ms
9	handleFunctionCalls_blockingTools_sequential_executesSerially	BlockingTool ×2 @ 500 ms	SEQUENTIAL	duration ≥ 1000 ms
10	handleFunctionCalls_blockingTools_parallel_preservesOrder	BlockingTool slow(800ms), fast(100ms)	PARALLEL	response[0] == slow tool
11	handleFunctionCalls_blockingTools_parallel_usesMultipleThreads	BlockingTool ×2 @ 500 ms	PARALLEL	thread1 ≠ thread2
12	handleFunctionCalls_nonBlockingTools_parallel_stillWorksCorrectly	EchoTool ×3	PARALLEL	correct responses in order (regression)

[hemasekhar-p]

Hi @YuqiGuo105, Thank you for your contribution! We appreciate you taking the time to submit this pull request. I have noticed The Maven build is failing due to test case failures in this PR. Could you please address these failure?

[YuqiGuo105]

Hi @hemasekhar-p, the failed test case is fixed.

Reason

Every pre-existing test that failed involved exactly one tool call. When the parallel path (concatMapEager + subscribeOn(Schedulers.io())) was applied unconditionally, a single tool's execution was still dispatched to an IO thread. This caused two async timing problems:

1. OTel span not yet ended — doFinally(span.end()) runs on the IO thread after blockingGet() returns on the main thread, so the test reads spans before they're exported.
2. Error not yet delivered — the tool error propagates asynchronously on the IO thread, arriving after the test's assertError() already ran (seeing errors=0).

Fix

if (invocationContext.runConfig().toolExecutionMode() == ToolExecutionMode.SEQUENTIAL
    || validFunctionCalls.size() <= 1) {

The || validFunctionCalls.size() <= 1 guard routes single-tool calls to concatMapMaybe, which runs entirely on the calling thread — no IO dispatch, no async race.

Since parallel vs. sequential is meaningless for a single task, this is semantically correct and has no functional impact.