Improve enforced type coverage in CI

[tamird]

The SDK already runs mypy in CI, but much of the public surface is still only partially annotated. I would like to improve type coverage in a way that is enforced by CI, with priority on the APIs consumers interact with directly.

The main goal is not to make a large mechanical annotation-only change. It is to tighten the type contract incrementally while preserving runtime behavior and avoiding broad escape hatches such as Any, cast, or type: ignore/attribute hacks where a real type can be expressed.

Proposed direction:

1. Start with consumer-facing modules where annotations provide the most value:

   • Web API clients and responses, sync and async
   • Webhook clients and responses
   • public error types
   • model serialization helpers used to build Slack API payloads

2. Add a small shared JSON type vocabulary for payload-like values, for example:

   • JSONScalar = Optional[Union[str, int, float, bool]]
   • JSONDict = Dict[str, JSONValue]
   • JSONList = List[JSONValue]
   • JSONValue = Union[JSONScalar, JSONDict, JSONList]

   This gives model serialization and request-building code something more precise than raw dict/Any, while still matching the JSON-ish data the SDK sends and receives.

3. Tighten CI in stages rather than flipping full strict = true immediately. A practical first step could enable checks that are useful and relatively low-risk across the current package, such as:

   • check_untyped_defs = true
   • warn_return_any = true
   • warn_redundant_casts = true
   • warn_unused_configs = true
   • strict_equality = true
   • disallow_subclassing_any = true
   • no_implicit_reexport = true
   • enable_error_code = "ignore-without-code"

4. Enable disallow_untyped_defs first for selected public/consumer-facing modules. After that is green, expand the list module-by-module instead of adding a large unprincipled exclusion list.

5. Keep any remaining exclusions explicit and temporary. For example, if SCIM or legacy RTM need separate treatment, that should be called out as a known follow-up rather than hidden behind a broad ignore block.

6. Continue to use the existing project workflow:

   • update canonical Web client sources, not generated files directly
   • run codegen for generated Web clients
   • validate with the existing mypy/lint/test scripts

Open questions for maintainers:

• Is this type-hardening direction welcome for this repository?
• Which public modules should be prioritized first?
• Are there areas, such as legacy RTM or SCIM, that maintainers would prefer to leave out of the initial pass?
• Is adding shared JSON payload aliases acceptable, or would you prefer a different naming/location for those types?

Category (place an x in each of the [ ])

• slack_sdk.web.WebClient (sync/async) (Web API client)
• slack_sdk.webhook.WebhookClient (sync/async) (Incoming Webhook, response_url sender)
• slack_sdk.models (UI component builders)
• slack_sdk.oauth (OAuth Flow Utilities)
• slack_sdk.socket_mode (Socket Mode client)
• slack_sdk.audit_logs (Audit Logs API client)
• slack_sdk.scim (SCIM API client)
• slack_sdk.rtm (RTM client)
• slack_sdk.signature (Request Signature Verifier)

[tamird]

Clarifying the above: I am happy to contribute work toward this. I would appreciate a indication of willingness to accept these changes before starting work.