fix: narrow DispatchOptions.upgrade type to string | null

[mcollina]

Summary

Fixes #5466

DispatchOptions.upgrade accepted boolean in the TypeScript type, but the runtime validator in lib/core/request.js rejects any truthy non-string value with InvalidArgumentError: upgrade must be a string. A TypeScript-valid call like client.dispatch({ path: '/', method: 'GET', upgrade: true }, handler) therefore fails at runtime.

Changes

• types/dispatcher.d.ts: Narrowed upgrade from boolean | string | null to string | null
• test/types/dispatcher.test-d.ts: Added type-level assertion that upgrade: true is rejected via expectNotAssignable, plus positive assertions for upgrade: 'websocket' and upgrade: null

Testing

• expectNotAssignable<Dispatcher.DispatchOptions>({ ..., upgrade: true }) — fails at type-check time
• Existing valid usage (upgrade: 'websocket', upgrade: null) remains accepted

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.45%. Comparing base (ce7bf4f) to head (1e125f0).
⚠️ Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5485      +/-   ##
==========================================
- Coverage   93.46%   93.45%   -0.01%     
==========================================
  Files         110      110              
  Lines       37124    37147      +23     
==========================================
+ Hits        34698    34716      +18     
- Misses       2426     2431       +5     

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