Fix parsing of empty long option values

[actuallyhrishikesh]

Fixes #958.

This change treats the presence of = in a long option as an explicit attached value, including empty values.

For example:

--output=

is now parsed as an option with an empty string value rather than being treated as a bare flag.

Tests:

• Added tokenizer coverage for empty long option values.
• Added an end-to-end regression test for --output=.
• Ran the full test suite and formatting checks.

Compatibility note: --flag= is now treated as a flag with an explicit empty attached value, consistent with the existing single-dash -f= behavior and the issue's explicit = rule.