refactor(query): remove proxy table engine

[sundy-li]

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

Summary

Remove the obsolete PROXY table engine now that the functionality can be replaced by materialized views. This removes the engine implementation, parser entry, storage factory registration, PROXY-specific settings, benchmark scripts, and the obsolete PROXY routing benchmark workflow job. The former routing coverage is preserved as materialized-view cost tests that verify choosing MV candidates by matching cluster-key predicates.

Tests

• Unit Test
• Logic Test
• Benchmark Test
• No Test - Explain why

Validation run:

• cargo fmt --all
• cargo check -p databend-common-ast -p databend-common-settings -p databend-common-storages-basic -p databend-common-storages-factory -p databend-common-storages-fuse
• cargo check -p databend-query
• cargo check -p databend-common-storages-basic --tests
• cargo test -p databend-common-settings --test it setting::test_set_settings -- --exact --test-threads=1
• UPDATE_GOLDENFILES=1 cargo test -p databend-query --test it storages::system::test_engines_table -- --exact --test-threads=1
• cargo test -p databend-query --test it storages::system::test_engines_table -- --exact --test-threads=1

Note: full cargo test -p databend-common-settings --test it was also tried, but existing singleton/config ordering in that test binary caused unrelated failures outside this change.

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):

AI assistance

• AI usage: Assisted with removing the PROXY table engine implementation, SQL/parser/storage/settings wiring, related tests, benchmark scripts, workflow cleanup, formatting, and validation.
• Responsible human: @sundy-li
• The responsible human has read every line of this diff and can explain each change

---

This change is 

[github-actions]

🤖 CI Job Analysis

Workflow: 33976917240

📊 Summary

• Total Jobs: 91
• Failed Jobs: 3
• Retryable: 0
• Code Issues: 3

❌ NO RETRY NEEDED

All failures appear to be code/test issues requiring manual fixes.

🔍 Job Details

• ❌ linux / test_unit: Not retryable (Code/Test)
• ❌ linux / PROXY routing benchmark (prefix): Not retryable (Code/Test)
• ❌ linux / PROXY routing benchmark (statistics): Not retryable (Code/Test)

---

🤖 About

Automated analysis using job annotations to distinguish infrastructure issues (auto-retried) from code/test issues (manual fixes needed).