fix: propagate policy parse errors in string adapter LoadPolicy

[zjncs]

Description

stringadapter.Adapter.LoadPolicy discards the error returned by persist.LoadPolicyLine:

for _, str := range strs {
    if str == "" {
        continue
    }
    _ = persist.LoadPolicyLine(str, model)
}

A malformed line (e.g. an unterminated quoted field: p, alice, data1, "read) is therefore silently skipped: LoadPolicy returns nil, the model simply lacks the rule, and Enforce returns false with no error — the caller has no way to notice the typo'd policy.

The file adapter propagates the same error (persist/file-adapter/adapter.go loadPolicyFile returns handler(line, model) errors), and LoadPolicyLine/LoadPolicyArray return errors precisely so callers can report malformed policies (see also #1106). This PR aligns the string adapter with that contract by returning the error.

Testing

• Added Test_LoadPolicyMalformedLine in persist/string-adapter/adapter_test.go: an adapter line with an unterminated quoted field must make LoadPolicy return an error.
  • Before the fix: FAIL — LoadPolicy() error = nil, want a parse error for the unterminated quoted field
  • After the fix: PASS
• All existing tests in the package (Test_KeyMatchRbac, Test_SavePolicyRoundTripWithCommas, Test_StringRbac) still pass, and the full go test ./... suite passes (8 packages, no failures).
• go vet ./persist/string-adapter/ clean.

Behavior change is limited to inputs that previously produced a silently-wrong model; well-formed input loads exactly as before.

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This PR was prepared with AI assistance (GitHub Copilot / ZCode autonomous agent). All findings were verified manually against the codebase before submission.