Add is_empty structural emptiness check for OutputSchema

[NISH1001]

Summary

• Add akd.utils.is_empty(value) — recursive structural emptiness check. Returns True for None, empty/whitespace strings, empty bytes, and containers (BaseModel, dict, list, tuple, set, frozenset) whose contents are all recursively empty. Concrete scalars like 0, False, 0.0, datetime(...) are NOT considered empty.
• Expose OutputSchema.is_empty() as a thin wrapper so callers can do result.is_empty() instead of hand-rolling per-schema checks. Useful for short-circuiting downstream logic when an agent/tool returns a structurally blank result.
• Lazy-import of akd.utils inside the method to avoid a circular import via akd/__init__.py.

Test plan

• uv run pytest tests/test_utils_is_empty.py -v — 43 passed
• Manual sanity: O().is_empty() True; O(a='hi').is_empty() False; O(a=' ').is_empty() True
• uv run pytest — full suite green

[sanzog03]

lgtm

[github-actions]

❌ Tests failed (exit code: 1)

📊 Test Results

• Passed: 593
• Failed: 2
• Skipped: 39
• Warnings: 182
• Coverage: 76%

Branch: feature/output-emptiness
PR: #431
Commit: 53fd4e1

📋 Full coverage report and logs are available in the workflow run.