feat(testmanagement): template selection + multi-select custom fields

[gaurav-singh-9227]

What & why

Closes a customer blocker in the Test Management createTestCase/updateTestCase tools. Two gaps:

1. No template selection — every test case was created with the default "Test Case Steps" template; no way to pick a custom template.
2. custom_fields couldn't hold multiple values — the Zod schema restricted values to string | number | boolean, blocking arrays, so multi-select custom fields couldn't be set programmatically.

How the contract was determined

I probed the live TM v2 API (a project with real custom fields incl. a field_multi_dropdown) before writing code:

• Template → API keys on the internal slug. Sending test_case_bdd was recognized (HTTP 422 "Scenario is required for BDD test cases"). A display name ("Test Case BDD") was silently ignored and fell back to test_case_steps. form-fields-v2 does not enumerate templates and there's no template-list endpoint, so resolving display→slug server-side isn't possible. Pass-through of the slug is the only viable approach.
• Unrecognized template values fail silently (HTTP 200, falls back to default) — so we now warn when the returned template differs from the requested one.
• template is create-only. The PATCH/update endpoint ignores it (even a valid slug didn't switch the template), so template is intentionally not added to updateTestCase.
• Multi-select works on create and update when keyed by field name with an array: { "Field Name": ["opt1","opt2"] }. Keying by field id or a CSV string is silently dropped.

Changes

• create-testcase.ts: optional template param (slug, pass-through) + a mismatch warning when the API silently falls back; custom_fields widened to accept arrays (multi-select). New exported CustomFieldValue type + customFieldValueSchema.
• update-testcase.ts: custom_fields widened to accept arrays. No template (API ignores it on update).
• testmanagement.test.ts: +4 tests — slug pass-through, the silent-fallback warning, and array custom_fields on both create and update. (apiClient mock gained patch.)

Testing

npm run build passes locally — lint, format, 186 tests (4 new), and tsc all green.

Known limitation

Switching an existing test case's template via updateTestCase is not supported by the TM API — it ignores the template field on PATCH. Template can only be set at creation; supporting post-creation switching would need a TM backend change.

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