Add rocksdb_iter_is_key_pinned and rocksdb_iter_is_value_pinned to C API

[sachinsharma3191]

Summary

• Adds two new C API functions to check whether an iterator's key or value is pinned:
  • rocksdb_iter_is_key_pinned(const rocksdb_iterator_t*)
  • rocksdb_iter_is_value_pinned(const rocksdb_iterator_t*)
• Uses the Iterator::GetProperty mechanism with rocksdb.iterator.is-key-pinned / rocksdb.iterator.is-value-pinned
• Includes test coverage in db/c_test.c exercising both with and without pin_data

Test plan

• c_test passes with the new iter_pinned test phase
• Tests verify pinned status with pin_data=true and exercises the functions without pin_data to ensure no crashes
• Follows existing C API conventions (returns unsigned char, const iterator parameter)

Closes #14903

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