Add bounds checks to StringHelper::format placeholder handling

[jmestwa-coder]

Summary

Add bounds validation to StringHelper::format to safely handle malformed or under-supplied placeholders.

The previous implementation indexed pattern[i + 1], pattern[i + 2], and params[arg] without validating bounds, which could trigger invalid memory access and process failure for malformed format strings or missing arguments.

Changes

• Replaced sentinel-style string iteration with length-checked iteration
• Added bounds checks before reading placeholder characters
• Added argument index validation before accessing params[arg]
• Preserved unresolved placeholders literally when arguments are missing

Tests

Added regression coverage for:

• empty format patterns
• missing placeholder arguments