refactor(test): centralize integration test mocks (#421)

## Describe your changes

Refactoring of how integration test mocks are managed. We are moving
from a scattered, directory-based mock structure to a centralized,
single-file approach using `.test.json` files co-located with their
corresponding tests.

**Key Changes:**
1. **Centralized Mocks:** A new `*.test.json` file now holds all mock
data (input, output, API calls, etc.) for a single test, making tests
easier to read and maintain.
2. **New Fixture Providers:** We've refactored `NangoActionMock` to use
a strategy pattern for loading mocks (now called "Fixture Providers"),
ensuring full backward compatibility while enabling the new system.
    *   `LegacyFixtureProvider`: For old tests.
* `UnifiedFixtureProvider`: For new, centralized `.test.json` files.
* `RecordingFixtureProvider`: A new utility to automatically migrate old
tests to the new format.
3. **Async Loading:** The mock loading process is now asynchronous to
avoid synchronous I/O operations, with `getFixtureProvider` returning a
promise.
4. **Strict Error Handling:** The new provider enforces strict error
handling for missing critical mock data (like `batchSave` or `output`),
matching the behavior of the legacy system.
5. **Automated Migration:** A new workflow allows developers to
automatically generate the new `.test.json` files by running existing
tests with a `MIGRATE_MOCKS=true` environment variable.

This new structure improves the developer experience by making test data
easier to manage and reason about, and it provides a simple, automated
path for migrating existing tests.

## Issue ticket number and link

[NAN-4463](https://linear.app/nango/issue/NAN-4463/simplify-mock-file-structure-in-the-nango-integrations-folder)

## Checklist before requesting a review (skip if just adding/editing
APIs & templates)

- [x] I added tests, otherwise the reason is: This is a refactoring of
the testing framework itself.
-   [ ] External API requests have `retries`
-   [ ] Pagination is used where appropriate
- [ ] The built in `nango.paginate` call is used instead of a `while
(true)` loop
- [ ] Third party requests are NOT parallelized (this can cause issues
with rate limits)
- [ ] If the sync requires metadata the `nango.yaml` has `auto_start:
false`
-   [ ] If the sync is a `full` sync then `track_deletes: true` is set
- [ ] I followed the best practices and guidelines from the [Writing
Integration
Scripts](/NangoHQ/integration-templates/blob/main/guides/WRITING_SCRIPTS.md)
doc

---

## Migration Guide

We have made migration as automated as possible. Follow these steps to
migrate an integration to the new structure.

### Step 1: Ensure Tests Pass
Before starting, make sure the existing tests for the integration are
passing using the legacy mocks.
```bash
npx vitest run integration-templates/integrations/<integration-name>/tests/
```

### Step 2: Run with Migration Mode
Run the tests again with the `MIGRATE_MOCKS` environment variable set to
`true`. This will trigger the `RecordingFixtureProvider`, which will
execute the tests against the old mocks and generate the new
`.test.json` files.
```bash
MIGRATE_MOCKS=true npx vitest run integration-templates/integrations/<integration-name>/tests/
```
*Note: You will see new `*.test.json` files appear next to your
`*.test.ts` files.*

### Step 3: Verify and Clean Up
1. **Run Tests Again:** Run the tests *without* the environment
variable. The system should now automatically pick up the new
`.test.json` files and use the `UnifiedFixtureProvider`.
    ```bash
npx vitest run
integration-templates/integrations/<integration-name>/tests/
    ```
2.  **Verify:** Ensure all tests pass.
3. **Delete Old Mocks:** Once confirmed, you can safely delete the old
mock folders for the migrated tests from the `mocks/` directory.

### Step 4: Troubleshooting
If a test fails after migration:
- **Check the JSON:** Open the generated `.test.json` and verify the
data looks correct.
- **API Matching:** If an API call isn't matching, check the `api`
section. The loader uses subset matching, so ensure the critical
parameters in the `request` object match what the code is sending.
- **Re-record:** You can delete the generated `.test.json` and re-run
Step 2 to re-record the interaction.

### Step 5: Commit
Commit the new `.test.json` files and the removal of the old `mocks/`
directories.

Author: agusayerza

integrations/.nango/schema.json

@@ -62636,5 +62636,5 @@
       "additionalProperties": false
     }
   },
-  "$comment": "This file was generated by Nango (v0.69.18)"
+  "$comment": "This file was generated by Nango (v0.69.20)"
 }

integrations/.nango/schema.ts

@@ -1,5 +1,5 @@
 // ---------------------------
-// This file was generated by Nango (v0.69.18)
+// This file was generated by Nango (v0.69.20)
 // You can version this file
 // ---------------------------