fix: reject invalid numeric flags

[NianJiuZst]

Summary

• validate number-typed flags during parseFlags
• reject empty, NaN, and infinite numeric inputs before commands build requests
• add parser coverage for invalid and valid number values

Root cause

Number flags were parsed with Number(value) and stored without validation. Invalid input such as --max-tokens abc became NaN, which could later serialize as null in request JSON or fail deeper in timeout handling.

Validation

• bun test test/args.test.ts
• bun run typecheck
• bunx eslint src/args.ts test/args.test.ts
• bun test

Note: full repository lint still has pre-existing failures in unrelated tests: test/auth/timeout-fix.test.ts and test/client/stream.test.ts.