fix(validation): report empty names/refs and survive nested input

[OGsiji]

Summary

Fixes #408. validation/validate.py silently skipped validation or crashed with a raw traceback on several inputs that the JSON Schema itself accepts — name, from, and to are strings with no minLength, so "" is schema-valid and reaches the semantic checks.

Bugs fixed

1. Empty-string names bypassed uniqueness checks. validate_unique_names collected names with a truthiness filter (if d.get("name")), so duplicate "" dataset/field/metric/relationship names were dropped before duplicate detection. Now filtered with is not None.
2. Empty-string relationship endpoints were never flagged. validate_references used if from_ds / if to_ds, so a relationship with from: "" or to: "" silently passed instead of being reported as an unknown dataset. Now is not None.
3. Deeply nested SQL crashed the validator. validate_sql_expression only caught ParseError/TokenError; pathologically nested expressions raised an uncaught RecursionError. Now caught and reported as a [SQL] diagnostic.
4. Deeply nested YAML crashed with a traceback. main() only caught yaml.YAMLError; deeply nested flow collections raise RecursionError during composition. Now caught, exits cleanly.

Tests

Added regression tests in both suites:

• validation/tests/test_validate.py (pytest): empty-name duplicates, missing-name skip, empty from/to reported, missing-endpoint skip, deeply nested SQL diagnostic.
• validation/test_validate.py (unittest): deeply nested YAML exits cleanly without a traceback, empty endpoint reported end-to-end.

All Validation CI steps pass locally on Python 3.11 and 3.12:

• uv run validation/test_validate.py — 41 passed
• pytest validation/tests/ — 55 passed
• uv run validation/validate.py examples/tpcds_semantic_model.yaml — PASSED

[kayemkim]

Thanks for picking this up the day after the issue went in, and welcome. Good call adding the cases to both suites, since validation-ci runs the unittest file and the pytest directory separately.

Ran the branch merged into current main (bc7b2c0) through the validation-ci steps on Python 3.11 to 3.14: all green. I reproduced the four #408 cases on main first: two datasets named "" and a relationship with to: "" print Validation PASSED with exit 0, and 5000 nested parentheses in an expression or 3000 nested [ in the file end in a raw RecursionError. On this branch the first two fail with [Unique] and [Reference] lines and the other two exit 1 with the new diagnostics. The next layer down, a custom_extensions dict nested 150 and 300 levels (valid YAML, deep jsonschema recursion), already fails cleanly with a [Schema] error on both trees, so I don't see a remaining gap of the same kind. The 11 Ossie documents under examples/ and converters/** keep their exit codes.

Two small things, neither blocking:

• In validate_sql_expression the first attempt now has except (ParseError, TokenError): pass followed by except Exception: pass; the second covers the first. The noqa: BLE001 markers are inert too, since validation/ has no ruff configuration.
• The new comments are denser than the rest of the file and mostly restate the PR description and test comments. The one-line style used nearby would read more evenly.

Ordering note: #271 (approved, conflicting with main) edits the same block in main(), and merging both conflicts in validate.py and the pytest file. Whichever lands second has a small rebase.

LGTM.