fix: Support SQL registry dump

[tandede]

What this PR does / why we need it:

feast registry-dump always instantiated the file-backed Registry directly. As a result, a valid SQL registry URL such as postgresql+psycopg://... was parsed as if it were an object-storage URI and rejected as an unsupported scheme before any registry data could be read.

This change routes registry-dump through FeatureStore.registry, the same backend-selection path used by the rest of Feast. SQL, Snowflake, remote, and file-backed registries therefore use their configured implementations, while the repository path is still supplied for relative file registry paths.

The regression test creates a real SQLite-backed SQL registry, writes an entity through SqlRegistry, and verifies that registry_dump() reads the entity back as JSON. The test fails on master with unsupported scheme sqlite and passes with this change.

Which issue(s) this PR fixes:

Fixes #6764

Checks

• I've made sure the tests are passing.
• My commits are signed off (git commit -s)
• My PR title follows conventional commits format

Testing Strategy

• Unit tests
• Integration tests
• Manual tests
• Testing is not required for this change

Tests and checks run:

• python -m pytest -q sdk/python/tests/unit/test_repo_operations_registry_dump.py sdk/python/tests/unit/infra/registry/test_sql_registry.py sdk/python/tests/unit/test_registry_string_config.py sdk/python/tests/unit/cli/test_cli_chdir.py (41 passed)
• python -m pytest -q -n 0 sdk/python/tests/unit/cli sdk/python/tests/unit/test_repo_operations_registry_dump.py sdk/python/tests/unit/test_repo_operations_validate_feast_project_name.py sdk/python/tests/unit/infra/scaffolding/test_repo_operations.py (15 passed)
• pre-commit run --files sdk/python/feast/repo_operations.py sdk/python/tests/unit/test_repo_operations_registry_dump.py
• mypy feast/repo_operations.py
• Manual pre-fix reproduction with a populated SQLite SQL registry, followed by successful JSON output on this branch

Misc

No documentation changes are needed because this restores the documented backend-independent behavior of registry-dump.