Add Phi4 test and fix regex parsing.

[pytorchbot]

Summary:

• Add Phi4 test
• Handle the invert flag properly
• Skip regex lookahead since it's not supported by RE2
• Catch std::exception instead of narrow json parser errors
• Make the special tokens arg optional in ObjC/Swift API

Differential Revision: D83641294

[pytorch-bot]

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🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/14821

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❌ 1 New Failure, 3 Cancelled Jobs

As of commit 6b5e08d with merge base e0dda90 ():

NEW FAILURE - The following job has failed:

• pull / unittest / windows / windows-job (gh)
  The process 'C:\Program Files\Git\cmd\git.exe' failed with exit code 128

CANCELLED JOBS - The following jobs were cancelled. Please retry:

• pull / test-llava-runner-linux / linux-job (gh)
  ##[error]The operation was canceled.
• pull / unittest / macos / macos-job (gh)
  ##[error]The operation was canceled.
• pull / unittest-editable / macos / macos-job (gh)
  ##[error]The operation was canceled.

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