feat: add Heap region support

[SahilJat]

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[x] I have read the Contributing Guide /Flagsmith/flagsmith/blob/main/CONTRIBUTING.md.
[ ] I have added information to docs/ if required so people know about the feature.
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[x] I have filled in the "How did you test this code" section below.

Changes

Closes #7730

The Heap integration was hardcoded to target https://heapanalytics.com , preventing users with European Heap instances ( https://eu.heapanalytics.com ) from using the integration.

This PR adds base_url support to the Heap integration, following the same pattern as the existing Amplitude integration:

• api/integrations/heap/constants.py — New file with DEFAULT_HEAP_API_URL constant.
• api/integrations/heap/heap.py — Use config.base_url instead of the hardcoded HEAP_API_URL constant.
• api/integrations/heap/serializers.py — Expose base_url in the API serializer fields.
• frontend/common/stores/default-flags.ts — Add base_url field to the Heap integration UI configuration.

This change is fully backwards compatible — existing configurations receive the default US URL via the data migration, and new configurations without an explicit base_url also default to US.

How did you test this code?

Unit tests

Added and updated tests in:

• api/tests/unit/integrations/heap/test_unit_heap.py :
• Existing test now asserts default URL resolves to https://heapanalytics.com/api/track .
• New test test_heap_wrapper__eu_base_url__uses_eu_url verifies EU URL is used correctly.
• api/tests/unit/integrations/heap/test_unit_heap_views.py :
• Updated list test to assert base_url is present in API response with default value.
• Updated create test to verify default base_url is set when omitted.
• New test test_create_heap_config__with_eu_base_url__returns_created verifies creating a config with a custom EU base URL.

Manual verification

1. Start the API: cd api && make serve

2. Create a Heap config without base_url — confirm it defaults to https://heapanalytics.com :
   curl -X POST http://localhost:8000/api/v1/environments/<ENV_KEY>/integrations/heap/
   -H "Authorization: Token "
   -H "Content-Type: application/json"
   -d '{"api_key": "test-key"}'

3. Create a Heap config with EU base_url — confirm it is stored and returned:
   curl -X POST http://localhost:8000/api/v1/environments/<ENV_KEY>/integrations/heap/
   -H "Authorization: Token "
   -H "Content-Type: application/json"
   -d '{"api_key": "test-key", "base_url": "https://eu.heapanalytics.com"}'

4. Start the frontend ( cd frontend && ENV=local npm run dev ) and navigate to Integrations → Heap Analytics — confirm the "Base URL" field is visible alongside the API Key field.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.39%. Comparing base (19ffc62) to head (bca1eb5).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7732      +/-   ##
==========================================
- Coverage   98.54%   98.39%   -0.15%     
==========================================
  Files        1452     1453       +1     
  Lines       55793    55818      +25     
==========================================
- Hits        54983    54924      -59     
- Misses        810      894      +84     

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[Zaimwa9]

@gemini-code-assist review

[gemini-code-assist]

Code Review

This pull request introduces a configurable base_url for the Heap integration, allowing users to specify custom endpoints (such as for EU-based Heap instances) instead of relying on a hardcoded default. The changes include backend model updates, serializers, database migrations, unit tests, and frontend configuration. The reviewer's feedback focuses on ensuring data integrity by explicitly enforcing null=False and blank=False on the new base_url field in both the model and migration, as well as safely backfilling soft-deleted records during the migration using the all_objects manager.

[Zaimwa9]

Overall we are able to drop the database migration. I guess this was inspired by the amplitude but with heap the url field is null for all the integrations and previous records will still be using the same url.
So self.url = "{config.base_url or DEFAULT_HEAP_API_URL}/api/track" should do the job.

We are still discussing internally how to address the frontend so a first step would be to focus on the backend. We will discuss and come back to you

[Zaimwa9]

Thanks. Ideally just adding a coverage test asserting the request is made with the right url

[matthewelwell]

Approving based on @Zaimwa9 's review, and we will add his suggested test in a separate PR. Thanks for the contribution @SahilJat .

See note here: #7732 (review)