test(sveta): Write API integration tests for calculator endpoints

Outcome: done
Changed: tests/test_api.py:53-68,75-85,90-98 — added clamp low>high, missing-field, and subtract missing-field tests; src/typosaurus_sandbox/presentation/api/v1/calculator.py:2,51-55 — added HTTPException import and ValueError catch in calculate_clamp
Verified by: `make verify` — exit 0, 47 tests OK, zero warnings, zero compile errors
Findings:
- tests/test_api.py now contains 22 integration tests covering all five endpoints with success, validation errors (missing fields, wrong types), and the clamp low>high ValueError boundary.
- clamp low>high error is returned as 422 via HTTPException in the route handler, matching the acceptance criterion.
- All 47 tests (25 unit + 22 integration) pass with zero warnings.
Open: none
Confidence: high — all acceptance criteria addressed, all tests pass, verification gate passed

Typosaurus-Run: d4f7c095ea9d49c69663ae6d01a21513
Typosaurus-Node: 9fe1a5bb3c164f6eb359366ca55b1067
Typosaurus-Agent: @sveta
Refs: #15

src/typosaurus_sandbox/presentation/api/v1/calculator.py

@@ -1,6 +1,6 @@
 # retoor <retoor@molodetz.nl>
 
-from fastapi import APIRouter
+from fastapi import APIRouter, HTTPException
 from pydantic import BaseModel, Field
 
 from typosaurus_sandbox.domain.calculator import add, clamp, clamp_to_byte, subtract
@@ -48,7 +48,11 @@ def calculate_subtract(body: SubtractRequest) -> IntResult:
 
 @calculator_router.post("/clamp", response_model=FloatResult)
 def calculate_clamp(body: ClampRequest) -> FloatResult:
-    return FloatResult(result=clamp(body.value, body.low, body.high))
+    try:
+        result = clamp(body.value, body.low, body.high)
+    except ValueError:
+        raise HTTPException(status_code=422, detail="low must not exceed high")
+    return FloatResult(result=result)
 
 
 @calculator_router.post("/clamp-to-byte", response_model=IntResult)