feat(api): add GET /forms/{submission_id} endpoint with tests

[aryan-2255]

This PR adds a new read endpoint to retrieve an existing form submission by ID, completes the missing retrieval path for form records, and adds focused tests to validate success and error behavior.

##What Changed -

1.) API Route

Added a new endpoint in api/routes/forms.py:
GET /forms/{submission_id}
Returns 200 with the submission payload when found
Returns 404 with a clear message when not found

2.) Repository Layer

Added a repository helper in api/db/repositories.py:
get_form_submission(session, submission_id)
direct primary-key lookup for efficient retrieval

3.) Tests

Replaced placeholder form tests with real endpoint tests in tests/test_forms.py:
test_get_form_submission_by_id
test_get_form_submission_by_id_not_found
Test design avoids external LLM/Ollama dependency by monkeypatching controller fill behavior, so tests remain fast and deterministic.

Behaviour Before vs After -

Before

Submissions could be created via POST /forms/fill
No API endpoint existed to fetch a submission later by ID

After

Submissions can now be retrieved via GET /forms/{submission_id}
Clients can reliably fetch generated output metadata (including output_pdf_path)
Missing records return a consistent 404 response

##API Contract

GET /forms/{submission_id}
200 OK: returns FormFillResponse payload
404 Not Found: returns error detail "Form submission not found"

##Why This Change Matters

Completes a core CRUD read path for form submissions
Enables frontend/history/detail views to load a submission directly
Improves operational usability for follow-up access to generated form outputs
Keeps architecture aligned with existing repository + route separation

##Impact Assessment

Backward compatibility: Fully backward-compatible (additive endpoint only)
Risk level: Low
Performance impact: Minimal; single-row lookup by primary key
External dependency impact: None

##Validation Performed

Ran local test suite:
source .venv/bin/activate && PYTHONPATH=. pytest -q
Result: 3 passed

##Files Included in This PR
api/routes/forms.py
api/db/repositories.py
tests/test_forms.py

##Out of Scope
No changes to fill pipeline logic
No schema model restructuring
No Docker/runtime config changes

closed #165

[Copilot]

Pull request overview

This PR adds a read endpoint to retrieve an existing form submission by ID, completing the “GET by id” path for form submissions and adding tests to validate success and not-found behavior.

Changes:

• Added GET /forms/{submission_id} to return a FormFillResponse or a 404 when missing.
• Added a repository helper get_form_submission(session, submission_id) using a primary-key lookup.
• Replaced placeholder form tests with real endpoint tests for success and not-found cases.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
api/routes/forms.py	Adds the GET submission-by-id route and wires it to repository lookup + 404 behavior.
api/db/repositories.py	Adds get_form_submission() helper for direct primary-key retrieval.
tests/test_forms.py	Adds endpoint tests for retrieving a submission and for the 404 not-found case.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[Copilot]

fill_form raises AppError, but api/main.py never registers register_exception_handlers, so AppError will surface as an unhandled exception (500) rather than the intended 404. Since this file now introduces HTTPException-based 404s, consider standardizing by switching the existing AppError("Template not found") path to HTTPException(status_code=404, detail=...), or alternatively register the AppError handler in api/main.py and align the error response shape (currently {"error": ...} vs {"detail": ...}).

[Copilot]

The test uses template_response.json()["id"] without asserting the template creation succeeded. Adding assert template_response.status_code == 200 (and similarly for fill_response) will make failures clearer and avoid misleading KeyError/JSONDecodeError when the request fails.

Suggested change

	template_response = client.post("/templates/create", json=_template_payload("Form Template"))
	template_id = template_response.json()["id"]
	fill_response = client.post(
	"/forms/fill",
	json={"template_id": template_id, "input_text": "test transcript"},
	)
	template_response = client.post("/templates/create", json=_template_payload("Form Template"))
	assert template_response.status_code == 200
	template_id = template_response.json()["id"]
	fill_response = client.post(
	"/forms/fill",
	json={"template_id": template_id, "input_text": "test transcript"},
	)
	assert fill_response.status_code == 200