tool_getparam: simplify the --rate parser

[bagder]

Avoids a memcpy()

[Copilot]

Pull request overview

This PR refactors the --rate parameter parser in set_rate() to simplify the parsing logic and eliminate an unnecessary memory copy operation. The changes replace a two-step approach (using strchr() to find the delimiter, then memcpy() to extract the number) with direct pointer-based parsing using curlx_str_number() and curlx_str_single().

• Eliminates intermediate buffer allocation and memcpy() call
• Changes denominator type from long to curl_off_t for consistency with parsing functions
• Streamlines the parsing flow by using pointer advancement built into the parsing functions

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