chore: fix mypy warnings in tests/api

What does this merge request do and why?

This MR fixes all mypy warnings on tests/api folder.

# Remove `ai_gateway/experimentation` from https://gitlab.com/gitlab-org/modelops/applied-ml/code-suggestions/ai-assist/-/blob/main/Makefile#L20
❯ make check-mypy
--snip--
Running mypy check...
tests/api/v3/test_v3_code.py:324: error: Invalid index type "str" for "str"; expected type "SupportsIndex | slice[Any, Any, Any]"  [index]
tests/api/v3/test_v3_code.py:700: error: Argument "endpoint" to "ModelMetadata" has incompatible type "str"; expected "Url"  [arg-type]
tests/api/v3/test_v3_code.py:948: error: Invalid index type "str" for "str"; expected type "SupportsIndex | slice[Any, Any, Any]"  [index]
tests/api/v2/test_v2_code.py:446: error: List comprehension has incompatible type List[Message]; expected List[dict[str, str]]  [misc]
tests/api/v2/test_v2_code.py:446: error: Argument "role" to "Message" has incompatible type "str"; expected "Role"  [arg-type]
tests/api/v2/test_v2_code.py:449: error: Dict entry 0 has incompatible type "str": "list[dict[str, str]]"; expected "str": "list[str]"  [dict-item]
tests/api/v2/test_v2_code.py:1329: error: Argument 1 to "set" of "ContextVar" has incompatible type "list[Never]"; expected "set[str]"  [arg-type]
tests/api/v2/test_v2_code.py:1373: error: Argument 1 to "set" of "ContextVar" has incompatible type "list[Never]"; expected "set[str]"  [arg-type]
tests/api/v2/test_v2_chat_agent.py:75: error: Incompatible return value type (got "AgentBaseEvent", expected "str")  [return-value]
tests/api/v2/test_v2_chat_agent.py:84: error: Missing named argument "model_metadata" for "AgentRequest"  [call-arg]
tests/api/v2/test_v2_chat_agent.py:106: error: Argument "endpoint" to "ModelMetadata" has incompatible type "str"; expected "Url"  [arg-type]
tests/api/v2/test_v2_chat_agent.py:164: error: Argument "endpoint" to "ModelMetadata" has incompatible type "str"; expected "Url"  [arg-type]
tests/api/v2/test_v2_chat_agent.py:173: error: Missing named argument "model_metadata" for "AgentRequest"  [call-arg]
tests/api/v2/test_v2_chat_agent.py:213: error: Argument 2 to "chunk_to_model" has incompatible type "type[AgentFinalAnswer]"; expected "AgentBaseEvent"  [arg-type]
tests/api/v2/test_v2_chat_agent.py:272: error: Argument "endpoint" to "ModelMetadata" has incompatible type "str"; expected "Url"  [arg-type]
tests/api/v2/test_v2_chat_agent.py:325: error: Argument 2 to "chunk_to_model" has incompatible type "type[AgentToolAction]"; expected "AgentBaseEvent"  [arg-type]
tests/api/v2/test_v2_chat_agent.py:341: error: Unexpected keyword argument "model_metadata" for "ReActAgentInputs"  [call-arg]
tests/api/v2/test_v2_chat_agent.py:366: error: Missing named argument "model_metadata" for "AgentRequest"  [call-arg]
tests/api/v2/test_v2_chat_agent.py:375: error: Missing named argument "model_metadata" for "AgentRequest"  [call-arg]
tests/api/v2/test_v2_chat_agent.py:385: error: Missing named argument "model_metadata" for "AgentRequest"  [call-arg]
tests/api/v2/test_v2_chat_agent.py:408: error: Missing named argument "model_metadata" for "AgentRequest"  [call-arg]
tests/api/v2/test_v2_chat_agent.py:436: error: Missing named argument "model_metadata" for "AgentRequest"  [call-arg]
tests/api/v2/test_v2_chat_agent.py:453: error: Missing named argument "model_metadata" for "AgentRequest"  [call-arg]
tests/api/v2/test_v2_chat_agent.py:471: error: Missing named argument "model_metadata" for "AgentRequest"  [call-arg]
tests/api/v2/test_v2_chat_agent.py:516: error: Missing named argument "model_metadata" for "AgentRequest"  [call-arg]
tests/api/v2/test_v2_chat_agent.py:630: error: Argument 2 to "chunk_to_model" has incompatible type "type[AgentFinalAnswer]"; expected "AgentBaseEvent"  [arg-type]
tests/api/v1/test_v1_search_docs.py:136: error: Function "tests.api.v1.test_v1_search_docs.request_body" is not valid as a type  [valid-type]
tests/api/v1/test_v1_search_docs.py:136: note: Perhaps you need "Callable[...]" or a callback protocol?
tests/api/v1/test_v1_prompts.py:71: error: Argument 1 to "len" has incompatible type "list[str] | None"; expected "Sized"  [arg-type]
tests/api/v1/test_v1_prompts.py:249: error: Argument 1 to "len" has incompatible type "list[str] | None"; expected "Sized"  [arg-type]
Found 29 errors in 5 files (checked 158 source files)
make: *** [Makefile:99: check-mypy] Error 1

How to set up and validate locally

1. Check out to this merge request's branch.
2. Ensure the following commands do not result in any warnings (they are already run as part of CI).

   make check-mypy
   make test

Merge request checklist

• Tests added for new functionality. If not, please raise an issue to follow up.
• Documentation added/updated, if needed.

Contributes to #715 (closed)