[rejected AI] Fix completion for attached option values

[err0r4eve3]

When an incomplete value is joined to a long option with =, _resolve_incomplete strips the option prefix so the parameter can complete only the value. Bash and Zsh replace the entire current word, though, so returning always for --color=al drops the option name.

Preserve the stripped prefix and reattach it to completion items only after the input resolves to an option that accepts a value. Rebuild each item with its original type, help, and custom metadata so custom completion behavior remains intact. Add regression coverage for both an empty and a partial attached value, plus a changelog entry.

Fixes #2847

Tests:

• uv run pytest
• uv run mypy
• uv run pre-commit run --files CHANGES.md src/click/shell_completion.py tests/test_shell_completion.py

[davidism]

https://palletsprojects.com/contributing/llm-ai