feat(postgres): add variable interpolation to postgres steps

[alileza]

Summary

• Add {{variable}} interpolation support to postgres handler steps, matching the behavior already present in the HTTP client handler
• Previously, variables captured via HTTP response steps (e.g. saved as "{{cr_id}}") could not be used in postgres steps — this made it impossible to cross-reference HTTP-created resources with direct database assertions

What changed

Three postgres handler methods now call ReplaceVariables() on their inputs:

• executeSQL — interpolates docstring content before executing
• setTableValues — interpolates cell values when inserting rows
• tableShouldContain — interpolates expected cell values when asserting

Example

This now works:

# Save an ID from HTTP response
When "api" sends "POST" to "/api/users"
Then "api" response json "id" saved as "{{user_id}}"

# Use it in a postgres assertion
Then "db" table "users" contains:
  | id           | status |
  | {{user_id}}  | active |

# Or in raw SQL
Given "db" executes:
  """
  UPDATE users SET verified = true WHERE id = '{{user_id}}'
  """

Test plan

• Verified go build ./... passes
• Manually tested with a project that uses HTTP + postgres steps together

[alileza]

Closing fork PR — reopening directly from org repo to fix CI permissions issue