chore: add support for gpt 5.1

[uinstinct]

Description

Support for the new gpt 5.1

resolves CON-4932

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Checklist

• [] I've read the contributing guide
• [] The relevant docs, if any, have been updated or created
• [] The relevant tests, if any, have been updated or created

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Tests

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Summary by cubic

Add support for gpt-5.1 with correct model metadata and inclusion in recommended models. Enables chat usage with a 128k context and 16,384 max completion tokens; addresses Linear CON-4932.

• New Features
  • Registered gpt-5.1 in the provider registry (display name, 128k context, 16,384 max completion).
  • Added gpt-5.1 to CometAPI recommended chat models.

^{Written for commit a5e93dd. Summary will update automatically on new commits.}

[cubic-dev-ai]

No issues found across 2 files

[RomneyDa]

See comments, also can we

• add to the add models form in GUI
• add it to recommended agent models i.e. make sure it gets multi edit tool
• add comments about how the model was tested e.g. does it do well in agent mode? both CLI and extensions?
• make sure it's added to auto tool support
• update the docs

[RomneyDa]

400 k context, 128 k max tokens