fix(core): parse Content-Type boundary parameters with MIME whitespace and escapes #2

Problem

boundary_from_content_type() currently rejects quoted boundary values containing valid quoted-pair escapes and does not tolerate optional whitespace after a quoted parameter value before the next ;.

Production-readiness risk

Real clients and proxies can emit MIME-compatible parameter formatting. The parser should be strict about invalid values, but compatible with valid quoted-string syntax.

Acceptance criteria

• Quoted boundary values can contain valid quoted-pair escapes.
• Optional whitespace after parameter values is accepted before the next parameter separator.
• Invalid unterminated or malformed quoted values still return None.
• Regression tests cover both accepted and rejected cases.