Use uint64 in C-API instead of string workaround

[CGodiksen]

This PR changes the C-API and C header to use unsigned 64-bit integers for the vacuum() and optimize() arguments instead of the previous workaround that used a string and converted the string to u64 in the C-API.

[Copilot]

Pull request overview

This PR updates the modelardb_embedded C-API (and its bindings) to accept optional uint64_t values for vacuum() retention period and optimize() target size, removing the prior string-based workaround.

Changes:

• Change the C-API functions modelardb_embedded_vacuum() and modelardb_embedded_optimize() to take const uint64_t* (nullable) instead of const char*.
• Update the Rust C-API implementation to read optional u64 values directly from pointers.
• Update the Python binding to pass uint64_t* values via cffi for these parameters.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
crates/modelardb_embedded/src/capi.rs	Switch FFI parameter types from C strings to *const u64 and remove string parsing.
crates/modelardb_embedded/bindings/python/modelardb/operations.py	Update Python cffi calls to pass uint64_t* instead of char[] strings.
crates/modelardb_embedded/bindings/c/modelardb_embedded.h	Update C header signatures to use const uint64_t* for the optional arguments.

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