fix(core): harden token amount validation

[FidelCoder]

Summary

Fixes #2090.

Replaces the decimal validation regex in convertToTokenAmount() with a linear validator to avoid ReDoS-style backtracking on long invalid input.

Changes

• Preserve valid integer, decimal, trailing-decimal, and negative amount behavior.
• Keep rejecting scientific notation and malformed numeric strings.
• Add regression coverage for long adversarial invalid input.

Tests

• git diff --check
• Not run: pnpm --filter @x402/core test -- test/unit/utils/utils.test.ts because local Node is v18.19.1 and pnpm requires Node v22.13+
• Not run: pnpm --filter @x402/core lint:check because local Node is v18.19.1 and pnpm requires Node v22.13+
• Not run: pnpm --filter @x402/core format:check because local Node is v18.19.1 and pnpm requires Node v22.13+
• Not run: pnpm --filter @x402/core build because local Node is v18.19.1 and pnpm requires Node v22.13+

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