[rejected AI] Fix shell completion for equals-separated option values

[LouisDeconinck]

Fixes #2847

Summary

• preserve the option prefix when completing values entered as --option=value
• add regression coverage for Bash and Zsh completion output
• document the fix in the unreleased changes

Tests

• uv run pytest -q
• `uv run ruff check src/click/shell_completion.py tests/test_shell_completion.py

[davidism]

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