refactor(errors): centralize custom errors

[tejaskash]

Summary

• move all custom error class definitions into src/errors
• export centralized errors through the errors barrel
• update implementations and tests to import errors from their new owner

Verification

• bun test (569 tests)
• bun run typecheck
• bun run lint:check
• bun run format:check
• bun run build

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.98%. Comparing base (a08c73f) to head (a6a407b).

Additional details and impacted files

@@            Coverage Diff            @@
##           refactor    #1885   +/-   ##
=========================================
  Coverage     95.98%   95.98%           
=========================================
  Files           208      207    -1     
  Lines          9636     9643    +7     
=========================================
+ Hits           9249     9256    +7     
  Misses          387      387           

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[notgitika]

LGTM