fix: Chatbot encoding and bytes errors

[kgabryje]

SUMMARY

Fixes two issues in the MCP chatbot/SQL execution flow:

1. Cached query results failing with bytes/encoding errors — The result cache was storing raw pandas DataFrames (which get pickled by some cache backends into bytes). On retrieval, the code assumed thedata was always a DataFrame, causing errors when encountering bytes. This PR serializes DataFrames to a JSON-safe dict format ({"records": [...], "columns": [...]}) before caching, and adds a _deserialize_cached_data() method that gracefully handles all formats (dict, DataFrame, None, and bytes as a cache miss) — intentionally avoiding pickle deserialization to prevent code-execution attacks from a compromised cache backend.
2. MCP middleware ordering was reversed — FastMCP 2.14+ changed how middleware is chained (uses reversed(middleware)), so the first-added middleware is now outermost. The middleware list ordering in server.py was updated to reflect this: GlobalErrorHandler → Logging → SizeGuard → Caching (outermost → innermost).

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

TESTING INSTRUCTIONS

1. Start Superset with MCP enabled
2. Run a SQL query via the MCP chatbot that returns results
3. Run the same query again — it should hit the cache and return correctly (no bytes/encoding errors)
4. Verify DML queries (INSERT/UPDATE) that return no data also work from cache
5. Verify middleware logging shows correct nesting order (error handler outermost)

ADDITIONAL INFORMATION

• Has associated issue:
• Required feature flags:
• Changes UI
• Includes DB Migration (follow approval process in SIP-59)
  • Migration is atomic, supports rollback & is backwards-compatible
  • Confirm DB migration upgrade and downgrade tested
  • Runtime estimates and downtime expectations provided
• Introduces new feature or API
• Removes existing feature or API

[bito-code-review]

Code Review Agent Run #b10c04

Actionable Suggestions - 0

Review Details

• Files reviewed - 2 · Commit Range: 521afb0..521afb0

  • superset/mcp_service/server.py
  • superset/sql/execution/executor.py
• Files skipped - 0
• Tools

  • Whispers (Secret Scanner) - ✔︎ Successful
  • Detect-secrets (Secret Scanner) - ✔︎ Successful
  • MyPy (Static Code Analysis) - ✔︎ Successful
  • Astral Ruff (Static Code Analysis) - ✔︎ Successful

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[codecov]

Codecov Report

❌ Patch coverage is 0% with 16 lines in your changes missing coverage. Please review.
✅ Project coverage is 63.68%. Comparing base (1867336) to head (521afb0).
⚠️ Report is 23 commits behind head on master.

Files with missing lines	Patch %	Lines
superset/sql/execution/executor.py	0.00%	11 Missing ⚠️
superset/mcp_service/server.py	0.00%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #38634      +/-   ##
==========================================
- Coverage   65.01%   63.68%   -1.34%     
==========================================
  Files        1817     2529     +712     
  Lines       72348   129281   +56933     
  Branches    23044    29790    +6746     
==========================================
+ Hits        47038    82334   +35296     
- Misses      25310    45484   +20174     
- Partials        0     1463    +1463     

Flag	Coverage Δ
hive	40.60% <0.00%> (?)
mysql	61.63% <0.00%> (?)
postgres	61.70% <0.00%> (?)
presto	40.62% <0.00%> (?)
python	61.97% <0.00%> (?)
sqlite	61.33% <0.00%> (?)

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