fix(mysql): infer LIMIT placeholders in prepare

[discord9]

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Closes #8142

What's changed and what's your intention?

This PR fixes MySQL prepared statements with placeholders in LIMIT/OFFSET, e.g.:

SELECT ... FROM t LIMIT ?

Before this change, DataFusion kept the placeholder in the logical plan but could not infer its type. GreptimeDB's existing MySQL prepare path only advertises placeholders with inferred concrete types, so the client saw the prepared statement as requiring 0 parameters and failed before execution with an argument count mismatch.

The changes are:

• Infer placeholders used by LogicalPlan::Limit (LIMIT/OFFSET) as Int64 in DfLogicalPlanner::get_inferred_parameter_types().
• This keeps the existing MySQL prepare/execute logic unchanged while making LIMIT/OFFSET placeholders typed.
• Add regression tests for LIMIT ? and mixed WHERE ? LIMIT ? prepared statements.

This does not change public APIs, schemas, or persisted data.

PR Checklist

Please convert it to a draft if some of the following conditions are not met.

• I have written the necessary rustdoc comments.
• I have added the necessary unit tests and integration tests.
• This PR requires documentation updates.
• API changes are backward compatible.
• Schema or data changes are backward compatible.

Tests:

• cargo fmt --check
• cargo test -p query -- test_get_inferred_parameter_types_limit_offset
• cargo test -p servers --features testing -- test_query_prepared

[gemini-code-assist]

Code Review

This pull request implements type inference for placeholders in LIMIT and OFFSET clauses, defaulting them to Int64, and ensures the MySQL handler correctly advertises all parameters to clients by using a null data type for unknown parameters. Review feedback identified a critical SQL injection vulnerability in the fallback execution path for unknown parameters, requiring string escaping. Additionally, the reviewer suggested improving error handling in the expression traversal logic to prevent swallowing potential errors.

[discord9]

Updated the PR to narrow the fix to the actual #8142 path: LIMIT/OFFSET placeholders are now inferred as Int64, and the MySQL handler/fallback string replacement changes were removed. The previous bot comments about unknown-placeholder fallback string substitution are therefore obsolete for the current diff.