fix: preserve shared option callback values

[suzunn]

Problem

Shared options that target the same parameter can process the same parser value more than once. When one option has a callback and another sibling option does not, the callback result can be overwritten by the sibling's raw parser value. In #2786 this allows an internal flag_value sentinel to reach the command callback when only the shortcut flag is provided.

Solution

I tightened the shared-parameter arbitration so a callback-less option with the same command-line source does not replace an existing processed value with a different raw value. This preserves the callback result while keeping the existing explicit-source ordering behavior for normal shared options.

I also added regression coverage for the flag-only case and both mixed ordering cases:

• --fetch keeps the callback result
• --custom typed --fetch lets the later flag win
• --fetch --custom typed lets the later explicit value win

Testing

• uv run --group tests pytest tests\test_options.py -k "shared_option_callback_value_is_preserved or default_dual_option_callback" -q
• uv run --group tests pytest tests\test_options.py -q
• uv run ruff check src\click\core.py tests\test_options.py
• uv run --group tests pytest -q
• git diff --check