Fix long option completion with equals

[sjh9714]

Shell completion currently strips the long option assignment prefix when completing values typed as --option=value. That makes bash return no replacement for partial values and makes zsh replace the whole option with the bare completion value.

This keeps the parsed value incomplete for matching, but remembers the --option= prefix and applies it back to plain completion values. File and directory completion markers are left unchanged so shell-native path completion still receives the raw path fragment. Space-separated option values and short options keep their existing bare value completions.

fixes #2847

Tests

• uv run pytest tests/test_shell_completion.py -k "long_option_equals" -q failed before the implementation with bare auto / always completions instead of --color=auto / --color=always.
• uv run pytest tests/test_shell_completion.py -k "long_option_equals" -q
• uv run pytest tests/test_shell_completion.py -q
• uv run pytest tests -q
• uv run ruff check src/click/shell_completion.py tests/test_shell_completion.py
• git diff --check

Note: I used Codex to help inspect the shell completion path and draft the regression tests, but I reviewed the final diff and ran the listed verification commands locally.