FLOGO-18450: Concurrent execution of parallel transition branches (#218)

* FLOGO-18450: Concurrent execution of parallel transition branches

Activities on parallel transitions (fan-out branches) previously executed
sequentially even though the flow models them as parallel. This adds opt-in
concurrent execution of ready branches, gated behind the existing
FLOGO_FLOW_CONCURRENT_TASK_EXECUTION env flag (default off).

When the flag is off the sequential DoStep loop is byte-for-byte unchanged, so
no existing flow regresses unless explicitly opted in. When on, FlowAction.Run
drives a new IndependentInstance.RunConcurrent worker pool that runs ready tasks
concurrently while preserving join semantics (a task runs only after all its
incoming links resolve) and order-independent results.

- support/env.go: GetConcurrentExecution(); instance/util.go
  IsConcurrentTaskExcutionEnabled delegates to it (one source of truth, shared
  with the change-tracker lock).
- Two-lock design on IndependentInstance: coarse stateLock (traversal/scheduling,
  taskInsts/linkInsts/subflows) + attrsLock RWMutex (shared scope + returnData),
  both nil in sequential mode. Lock order: stateLock -> changeTracker -> attrsLock.
- stepID/wiCounter/subflowCtr promoted to atomics.
- Per-task cancellable context (TaskInst.evalCtx) so context-aware activities
  abort on sibling failure.
- Drain-then-fail: first unhandled branch error cancels siblings, the pool waits
  for in-flight branches to return, then the global error handler fires once.
- execTask and the new execTaskConcurrent share extracted evalTaskBehavior +
  handleEvalResult helpers (removes duplication).
- Tests: unit + integration (parallelism, join-once, drain-then-fail, sequential
  parity), ~87% coverage of the new code; clean under -race. Also fixed
  pre-existing map-order-flaky TestGetErrorObject_* (def.Tasks()[0] ->
  def.GetTask("LogStart")).
- Benchmarks + a when-to-enable guide (instance/PARALLEL_BENCHMARKS.md).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

* update env var name

* add log statement when parallel execution of branches is enabled

* FLOGO-18450: address Copilot review (strict max-step, clear evalCtx, lock-order comment)

- RunConcurrent: increment stepCtr while holding coordMu (before Unlock) so the
  max-step safeguard is enforced strictly, matching the sequential loop; workers
  can no longer pop past maxStepCount.
- execTaskConcurrent: clear taskInst.evalCtx on exit so a TaskInst never retains a
  canceled group context after the pool tears down concurCtx; GoContext/GetGoContext
  fall back to the flow context on later reads.
- Correct the lock-hierarchy comment to stateLock -> changeTracker -> attrsLock (leaf),
  matching the actual order (the change tracker calls GetReturnData under its own lock).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

* FLOGO-18450: bump core to v1.6.20-rc.1 so the module builds out of the box

Production code uses trace.TagDefs/ParseTagDefs (added in #219) which only exist in
core v1.6.20-rc.1, but go.mod still pinned v1.6.17, so `go build ./...` failed with
"undefined: trace.TagDefs". Bumped core via `go get` + `go mod tidy`; tidy also
pruned now-unused transitive deps (x/net, go-redis, redsync, hashicorp/*, xxhash,
go-rendezvous) that the newer core no longer requires. Verified build on
windows/linux/darwin.

Also fixed a pre-existing broken reference in action_test.go, previously masked by
the TagDefs build failure: the dead RestartRequest test fixture referenced
flow/support.Interceptor (a type removed long ago); pointed it at
core/engine/support.Interceptor to match the canonical tester.RestartRequest so the
flow package's tests compile.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

* FLOGO-18450: document pre-existing TestTaskBehaviour failure (propagateSkip shadowing)

TestTaskBehaviour fails on `assert.True(t, skip)`. This is a pre-existing bug on
master, unrelated to FLOGO-18450 — it was hidden because model/simple could not
compile against the old pinned core (missing trace.TagDefs) and is exposed now that
the module builds.

Add an inline comment documenting the root cause (TaskBehavior.Skip's named return
`propagateSkip` shadows the package-level var and is never assigned, so Skip always
returns false; and the package var is init-cached so the test's os.Setenv is inert)
and how to fix it. No production behavior change here, per decision to track the fix
separately.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

* FLOGO-18450: pin core to v1.6.20 (stable) now that it is released

core v1.6.20 is now published (it is @latest and contains trace.TagDefs). Switch the
pin from the v1.6.20-rc.1 release candidate to the stable release via
`go get github.com/project-flogo/core@v1.6.20` + `go mod tidy`. No dependency-graph
change beyond the version/hash; build verified on windows/linux/darwin.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

Flogo 18589 fix error code (#221)

Flogo-18589: fix error code for validation and NumError error (#220)

Flogo-18285: custom tag support (#219)

Flogo-18471: fix mock error (#217)

FLOGO-16646: Added FlowInstanceID in error object (#214)

* Added activity type when typeID is null

* Added flowInstanceID in error object

Added activity type when typeID is null (#212)

Fixed formatting in log

Fix for FLOGO-16775

Fix for regression for timeout activity in subflow