Selector Signal fix with multiple workers

[yuandrew]

What was changed

Fixed a scenario where selectors without default branch was having shared data c.recValue overwritten. Also re-enables the SDK flag SDKFlagBlockedSelectorSignalReceive

Why?

Fix bug

Checklist

1. Closes Lost messages from workflow.NewChannel(ctx) #2066 and Signal sent to Selector can be lost if Default path blocks  #1624

2. How was this tested:
   Added new test

---

Note

Fixes selector receive logic to avoid losing channel items with multiple workers and changes UNBLOCK_SIGNAL_SELECTOR default handling; adds a targeted test.

• Workflow runtime:
  • Selector receive handling updated in internal/internal_workflow.go to only pre-store c.recValue when a default branch exists; otherwise defer storage in readyBranch to avoid races when multiple selectors block on the same channel. Uses SDKFlagBlockedSelectorSignalReceive gating.
• Flags:
  • Change unblockSelectorSignal default to os.Getenv("UNBLOCK_SIGNAL_SELECTOR") != "false" in internal/internal_flags.go (enabled unless explicitly set to "false").
• Tests:
  • Add TestChannelWorkerPattern in internal/internal_workflow_testsuite_test.go validating no values are lost with two workers consuming the same channel.

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