CLN: Consolidate decimal string parsing functions in tokenizer.c

[heoh]

• closes Consolidate decimal digit -> str functions in tokenizer.c #62717
• Tests added and passed if fixing a bug or adding a new feature
• All code checks passed.
• Added type annotations to new arguments/methods/functions.
• Added an entry in the latest doc/source/whatsnew/vX.X.X.rst file if fixing a bug or adding a new feature.

Description

This PR consolidates two similar string manipulation functions in tokenizer.c:

• _str_copy_decimal_str_c: handled decimal/thousands separator replacement with heap allocation
• copy_string_without_char: removed characters with caller-provided buffer

The refactored _str_copy_decimal_str_c now combines the strengths of both approaches:

• Uses caller-provided stack buffer (no heap allocation needed)
• Processes multiple character replacements
• Uses block memory operations (strspn, memcpy) instead of byte-by-byte processing
• Provides comprehensive validation and error handling

Changes

• Refactored _str_copy_decimal_str_c to write to caller-provided buffer instead of heap allocation
• Added helper functions and macros (str_consume_span, etc.) for efficient string parsing
• Removed copy_string_without_char function
• Updated str_to_int64 and str_to_uint64 to use the refactored function
• Simplified round_trip function with stack buffer allocation