refactor: remove unused OptionHelpExtra TypedDict from _click.types

[noQbot]

Pull Request

Discussion: #1937

Description

OptionHelpExtra in typer/_click/types.py is a TypedDict vendored from
Click 8.3.1. In Click it annotates the return value of Option.get_help_extra().
Typer's vendored _click never included that method, and Typer's own help-record
assembly (typer/core.py) does not use the type. As a result OptionHelpExtra
is referenced nowhere in the codebase and is not part of any public API surface:
it lives in the private typer/_click package and is not re-exported by
typer/__init__.py or typer/_click/__init__.py.

This PR:

• Deletes the unused OptionHelpExtra TypedDict.
• Removes the now-unused TypedDict import from typer/_click/types.py.

No runtime behavior or public-API change. If Typer later re-vendors Click's
get_help_extra() path, the type can be reintroduced alongside its caller.

AI Disclaimer

Models: Composer 2.5 (via Cursor + VinvAI )

Prompt (paraphrased): act on Vinv's dead-code finding for OptionHelpExtra —
confirm the symbol is genuinely unreferenced (no static references, no
re-export from any package __init__ or public API surface, no dynamic lookup
by name, and no get_help_extra caller in Typer's vendored _click), then
delete it along with any now-unused imports or types that existed solely to
support it.

AI transcript

• Vinv's dead-code sweep flagged OptionHelpExtra in typer/_click/types.py
  as unreferenced; the removal was implemented via Composer 2.5 (Cursor + VinvAI).
• Independently verified before finalizing:
  • Repo-wide grep for OptionHelpExtra → only the definition site matched.
  • Confirmed get_help_extra (the method it annotates in upstream Click) does
    not exist anywhere in Typer's vendored _click.
  • Confirmed it is not re-exported (typer/__init__.py,
    typer/_click/__init__.py, __all__) and not resolved dynamically.
  • Confirmed the TypedDict import had no other users.
• Deleted OptionHelpExtra and the now-unused TypedDict import.
• Ran python -m py_compile typer/_click/types.py → compiles clean. (Full
  pytest/coverage suite not run in this session.)

Checklist

• This PR links to a GitHub Discussion for the proposed code change.
• I added tests for the change.
• The new or updated tests fail on the main branch and pass on this PR.
• Coverage stays at 100%.
• The documentation explains the change if needed.

[svlandeg]

Closing, violates the guidelines on user contributions.