test(time): cover Time.UnmarshalJSON error path (100% coverage)

[justinhwang]

What

#208 added atomic.Time.MarshalJSON / UnmarshalJSON, but its TestTimeJSON only exercised valid JSON. The error branch of UnmarshalJSON (return err) was never executed:

func (x *Time) UnmarshalJSON(b []byte) error {
	var v time.Time
	if err := json.Unmarshal(b, &v); err != nil {
		return err   // <- never covered by #208's tests
	}
	x.Store(v)
	return nil
}

That dropped package coverage to 99.6%, below the 100% target in .codecov.yml.

Change

Adds an UnmarshalJSONError subtest to TestTimeJSON that feeds a well-formed JSON string which isn't a valid RFC 3339 timestamp. It asserts the decode error is propagated and that the wrapped value is left untouched.

Note: unlike the Duration/Bool/String error tests, this can't reuse the assertErrorJSONUnmarshalType helper — Time delegates to time.Time.UnmarshalJSON, which returns a *time.ParseError rather than a *json.UnmarshalTypeError.

Verification

• go test -cover . → 100.0% of statements (was 99.6%)
• make test (race suite) passes
• gofmt -s and go vet ./... clean

Test-only change; no CHANGELOG entry (not user-facing).

---

Before opening your pull request, please make sure that you've:

• updated the changelog if the change is user-facing; — n/a, test-only
• added tests to cover your changes;
• run the test suite locally (make test); and finally,
• run the linters locally (make lint). — gofmt/go vet clean; golint install fails on this Go toolchain (pre-existing, unrelated)

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (786022a) to head (6275acd).

Additional details and impacted files

@@             Coverage Diff             @@
##           master      #215      +/-   ##
===========================================
+ Coverage   99.51%   100.00%   +0.48%     
===========================================
  Files          23        23              
  Lines         410       410              
===========================================
+ Hits          408       410       +2     
+ Misses          1         0       -1     
+ Partials        1         0       -1     

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