fix(query): avoid task context for warehouse filter

[KKould]

I hereby agree to the terms of the CLA available at: https://docs.databend.com/dev/policies/cla/

Summary

• Avoid creating a full task QueryContext only to filter task messages by warehouse
• Use the same ClusterDiscovery::single_node_cluster(GlobalConfig) path that task context creation already uses for the local cluster
• Keep task execution sessions as SessionType::MySQL so task SQL behavior, including temporary table support, remains unchanged

This PR is scoped to reducing task scheduling overhead in warehouse filtering. It does not include the storage/cache accounting investigation or fix.

Tests

• Unit Test
• Logic Test
• Benchmark Test
• No Test - No SQL behavior change; validated with formatting and compile checks below

Validation:

• cargo fmt --check
• git diff --check
• cargo check -p databend-query

Type of change

• Bug Fix (non-breaking change which fixes an issue)
• New Feature (non-breaking change which adds functionality)
• Breaking Change (fix or feature that could cause existing functionality not to work as expected)
• Documentation Update
• Refactoring
• Performance Improvement
• Other (please describe):

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This change is 

[chatgpt-codex-connector]

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[KKould]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Hooray!

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