Fix sqlpage run_sql data mismatch error

[lovasoa]

Add parse_run_sql_variables to validate sqlpage.run_sql arguments and provide more descriptive error messages.

The previous error message "data did not match any variant of untagged enum SingleOrVec" was unhelpful when sqlpage.run_sql received invalid variable types. This change introduces specific error messages that identify the problematic variable and expected format.

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[cursor]

Cursor Agent can help with this pull request. Just @cursor in comments and I'll start working on changes in this branch.
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[lovasoa]

@cursoragent don't rewrite a custom parammap deserializer just for run_sql. Just improve the error messages both in the general parammap deserializer and attach precise descriptive human friendly context to the error in the run_sql call

[cursor]

Makes sense—I'll drop the bespoke parse_run_sql_variables and instead improve error reporting where ParamMap already deserializes:

• teach SingleOrVec’s Deserialize impl (or a helper used by it) to emit a friendlier message when the value isn’t a string/array of strings
• wrap the existing serde_json::from_str call in serde_path_to_error so we can attach the precise variable name/path when bubbling the error up from sqlpage.run_sql

That way we reuse the shared deserializer, and run_sql still gives clear, human-readable context without duplicating parsing logic.

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[lovasoa]

@cursoragent no new dependency allowed

[cursor]

Got it—I'll drop serde_path_to_error and rework the error reporting so it relies solely on our existing dependencies (e.g., enhance SingleOrVec’s custom Deserialize messages and add path/context manually in run_sql). Will update the PR shortly.

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