feat(postgres): add query result assertion steps

[alileza]

Summary

• Add two new postgres handler steps for executing arbitrary SQL queries and asserting on the result set
• Enables verifying JSONB fields, aggregations, filtered queries, and joined results in BDD tests

New steps

• "<resource>" query "<sql>" returns: — asserts exact match of returned rows (count + values)
• "<resource>" query result of "<sql>" contains: — asserts returned rows are a superset of the expected table

Example

# Exact match
Then "db" query "SELECT count(*) as cnt FROM orders" returns:
  | cnt |
  | 42  |

# Superset check — result may have more rows
Then "db" query result of "SELECT name, email FROM users" contains:
  | name  | email          |
  | Alice | alice@test.com |

Both steps support {{variable}} interpolation.

Test plan

• go build ./... passes
• Added 3 integration test scenarios in postgres.feature
• All postgres integration tests pass locally

[github-actions]

🔴 Integration Test Coverage: 32.0%

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