fix: avoid overwriting shared option callback results

[sjh9714]

Fixes #2786

Summary

When two options share the same parameter name, such as --custom and --fetch, invoking only --fetch could run its callback and then have that callback result overwritten by the uninvoked sibling option. In the reported case this exposed the internal sentinel value as $_fetch.

This updates parameter processing so that, once one option with a shared parameter name is invoked, uninvoked siblings with that same name are not processed later. Shared-name options with no invocation still follow the existing default handling, and multiple explicitly invoked options keep the existing invocation-order behavior.

I also added focused regression coverage for --fetch, --custom, both invocation orders, and the no-argument default case, plus a changelog entry.

Testing

• uv run pytest tests/test_options.py -k shared_dest_flag_callback_value_not_overwritten
  • Before the code change, the new regression failed for --fetch with custom=$_fetch.
  • After the code change, it passes.
• uv run pytest tests/test_options.py tests/test_basic.py
• uv run pytest
• uv run ruff check src/click/core.py tests/test_options.py
• uv run ruff format --check src/click/core.py tests/test_options.py
• uv run tox r
  • Local gap: this fails only in the py3.14t and stress-py3.14t environments on tests/test_utils.py::test_echo_via_pager. The same full py3.14t environment also fails on a clean origin/main checkout in my local setup, while the isolated uv run tox r -e py3.14t -- tests/test_utils.py::test_echo_via_pager command passes on clean origin/main.