Fix API handlers

[StpMax]

Description

In this PR, fixes were made for the correct handling of requests to the API handler of varying complexity: simple queries, queries with functions, queries with aggregate functions, with ORDER/GROUP BY, with LIMIT.

Fixes #FQE-2044

Type of change

(Please delete options that are not relevant)

• 🐛 Bug fix (non-breaking change which fixes an issue)
• ⚡ New feature (non-breaking change which adds functionality)
• 📢 Breaking change (fix or feature that would cause existing functionality not to work as expected)
• 📄 This change requires a documentation update

Verification Process

To ensure the changes are working as expected:

• Test Location: Specify the URL or path for testing.
• Verification Steps: Outline the steps or queries needed to validate the change. Include any data, configurations, or actions required to reproduce or see the new functionality.

Additional Media:

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Checklist:

• My code follows the style guidelines(PEP 8) of MindsDB.
• I have appropriately commented on my code, especially in complex areas.
• Necessary documentation updates are either made or tracked in issues.
• Relevant unit and integration tests are updated or added.

[entelligence-ai-pr-reviews]

Correctness: In APIResource.select, removing the logic that handles use_query_df and project_dataframe breaks support for aggregations (COUNT, SUM), GROUP BY, ORDER BY, and computed columns. Since filter_dataframe only performs a SELECT * on the fetched data, the final result will contain raw rows instead of the projected or aggregated values defined in the query. This logic must be restored to maintain SQL contract correctness.

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In `mindsdb/integrations/libs/api_handler.py` around the block after `result = filter_dataframe(...)`, restore the removed projection/aggregation logic. Reintroduce the `has_complex_targets`/`use_query_df` decision and call `query_df` for aggregations, GROUP BY, ORDER BY, or complex targets; otherwise use `project_dataframe`. Ensure WHERE and LIMIT handling matches the previous behavior so filters aren’t applied twice.

[StpMax]

such conditions handled later in SubSelect step