Fold ValidateEntryType into WithEntryType

[rdimitrov]

Summary

• Drop the standalone service.ValidateEntryType helper and inline the switch directly into setEntryType, so WithEntryType is the single validation site for entry type strings (addresses @blkt's review nit on Implement PUT /v1/entries/{type}/{name}/claims endpoint #720).
• Add an ErrInvalidEntryType sentinel so the handler can still map invalid input to HTTP 400 after validation moves inside the service layer.
• Simplify mapEntryType — drop the redundant pre-check and let the switch default return the same sentinel.

Fixes #736

Test plan

• task build
• task lint-fix — 0 issues
• task test — all unit and integration tests pass, including updated TestWithEntryType (asserts ErrorIs(err, ErrInvalidEntryType)) and the handler's unsupported entry type case (now exercises the service error path with a mock returning ErrInvalidEntryType).

🤖 Generated with Claude Code

[codecov-commenter]

Codecov Report

❌ Patch coverage is 90.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 60.63%. Comparing base (1cd9d2e) to head (c69b159).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/service/db/impl_entries.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #737      +/-   ##
==========================================
- Coverage   60.64%   60.63%   -0.01%     
==========================================
  Files         109      108       -1     
  Lines       10486    10480       -6     
==========================================
- Hits         6359     6355       -4     
+ Misses       3573     3572       -1     
+ Partials      554      553       -1     

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