[Firat17aze]

String::toInt() delegates to atol(), whose behavior is undefined when the input is outside the range of long. As a result, oversized values can produce different results across Arduino platforms. This fixes #175.

The conversion now accumulates decimal digits with explicit bounds checks. It returns 0 before overflow, accepts the exact LONG_MIN and LONG_MAX boundaries, preserves the existing leading-whitespace, sign, and trailing-text behavior, and avoids relying on global errno state.

Validation:

• Focused String::toInt() tests: 12 assertions in 7 test cases passed.
• Full host suite: 980 assertions in 309 test cases passed.
• AddressSanitizer and UndefinedBehaviorSanitizer full host suite: 980 assertions in 309 test cases passed.
• AVR-GCC 9.5 compiled api/String.cpp for ATmega328P with -Os -Wall -Wextra -Werror.
• git diff --check passed.

No hardware testing was performed; this is a conversion-only change covered by host tests and AVR compilation.

LLMs were used for review and code generation in some parts of this pull request.

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