THRIFT-5941: Add Ruby ext cppcheck coverage

[kpumuk]

Introduce cppcheck SCA for the Ruby native extension under lib/rb/ext and fix the existing findings so it can run in CI without noise.

Offenses addressed:

lib/rb/ext/compact_protocol.c:128:24: error: Shifting signed 32-bit value by 31 bits is undefined behaviour [shiftTooManyBitsSigned]
  return (n << 1) ^ (n >> 31);
                       ^
lib/rb/ext/compact_protocol.c:132:24: error: Shifting signed 64-bit value by 63 bits is undefined behaviour [shiftTooManyBitsSigned]
  return (n << 1) ^ (n >> 63);
                       ^
lib/rb/ext/compact_protocol.c:427:33: error: Signed integer overflow for expression '-(n&1)'. [integerOverflow]
  return (((uint32_t)n) >> 1) ^ -(n & 1);
                                ^
lib/rb/ext/struct.c:255:11: style: The scope of the variable 'key' can be reduced. [variableScope]
    VALUE key;
          ^
lib/rb/ext/struct.c:256:11: style: The scope of the variable 'val' can be reduced. [variableScope]
    VALUE val;
          ^
lib/rb/ext/struct.c:492:9: style: The scope of the variable 'i' can be reduced. [variableScope]
    int i;
        ^
lib/rb/ext/struct.c:531:9: style: The scope of the variable 'i' can be reduced. [variableScope]
    int i;
        ^
lib/rb/ext/struct.c:558:9: style: The scope of the variable 'i' can be reduced. [variableScope]
    int i;
        ^
lib/rb/ext/thrift_native.c:124:0: style: The function 'Init_thrift_native' is never used. [unusedFunction]
RUBY_FUNC_EXPORTED void Init_thrift_native(void) {
^

• Did you create an Apache Jira ticket? THRIFT-5941
• If a ticket exists: Does your pull request title follow the pattern "THRIFT-NNNN: describe my issue"?
• Did you squash your changes to a single commit? (not required, but preferred)
• Did you do your best to avoid breaking changes? If one was needed, did you label the Jira ticket with "Breaking-Change"?
• If your change does not involve any code, include [skip ci] anywhere in the commit message to free up build resources.