Fix numeric generation bounds and multipart header injection

[rasuvaeff]

Fix valid request-case generation at native numeric limits and reject unsafe multipart part headers.

• use overflow-safe integer quotient and product handling for integer multipleOf
• fail closed when a number multipleOf index cannot fit in the core integer generator
• move exclusive number bounds to the adjacent IEEE-754 value, including subnormals
• reject CR/LF and invalid field names in multipart content types and part headers

Validation:

• composer build (before the final test-only case)
• composer rector
• targeted Testo suites: RequestCaseArbitraryTest and SchemaArbitraryCompilerTest
• composer cs
• composer mutation ran and reported 91.62% MSI against the existing 92% threshold; it fails on the current package-wide baseline and is not hidden by this PR

[coderabbitai]

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