fix: preserve whitespace in equals-form flag values

[BenYang12]

What type of PR is this?

bug

What this PR does / why we need it:

parseFlags trimmed the entire argument before extracting the value after =. Consequently, --text=" hello " became " hello" even with TrimSpace: false, while --text " hello " preserved the value.

Extract the value from the original argument, keeping flag-name classification unchanged. Each flag's value parser now controls trimming and validation.

Which issue(s) this PR fixes:

Related to #2423, which inspired this investigation and fixed positional whitespace preservation, and #2169, which addressed string-slice trimming.

Testing

The regression tests fail in 25 cases with the original parser and all 60 cases pass with the fix. Coverage includes string, slice, and map flags; aliases; empty values; Unicode whitespace; embedded equals; and numeric/boolean validation.

Release Notes

Preserve whitespace in equals-form flag values, respecting each flag's trimming and validation rules. Numeric and boolean values with trailing whitespace now return parse
errors instead of being silently trimmed.

[dearchap]

LGTM. Verified manually:

• Change is minimal and correct: value extracted from raw token via strings.Cut(rargs[0], "=") while flag-name classification stays on the trimmed token. Raw prefix before the flag's = can only contain whitespace/- (never =), so the first = always maps correctly (embedded equals in values unaffected).
• Makes equals-form consistent with the separate-arg form, which already took the value raw.
• Focused behavior change for --int=42 / --bool= (parse errors instead of silent trim) is consistent with the separate-arg form and documented in release notes.
• Ran go build/vet/goimports (clean); new tests + full suite pass on PR head; re-applying tests onto base reproduces the ~20 equals-form whitespace failures, confirming they're meaningful regression tests.
• No API surface change; the docs/v3diff gate is unaffected.

Non-blocking nits: comment could note that firstArg is already trimmed (why raw arg is used); subtest name whitespace=%q prints _ for a space.