Add assistant.search Web API methods

[Cr1stal]

Summary

Adds Java SDK support for the Slack Web API assistant.search.* methods:

• assistant.search.context
• assistant.search.info

This includes request/response types, sync and async MethodsClient support, form serialization, rate-limit metadata, mock JSON fixtures, and local tests.

The assistant.search.context sample includes representative message, file, channel, and user result objects so downstream generated response types can infer concrete result shapes.

Related to slackapi/node-slack-sdk#2607.

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

• bolt (Bolt for Java)
• bolt-{sub modules} (Bolt for Java - optional modules)
• slack-api-client (Slack API Clients)
• slack-api-model (Slack API Data Models)
• slack-api-*-kotlin-extension (Kotlin Extensions for Slack API Clients)
• slack-app-backend (The primitive layer of Bolt for Java)

Testing

./mvnw -pl slack-api-client -Dtest=test_locally.api.methods.AssistantTest test
./mvnw -pl slack-api-client -Dtest=test_locally.api.MethodsTest,test_locally.sample_json_generation.MethodsResponseDumpTest,test_locally.api.methods.RequestFormBuilderTest test
./mvnw -pl slack-api-client -Dtest=test_locally.sample_json_generation.MethodsResponseDumpTest#assistant_search test
git diff --check

Requirements

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[salesforce-cla]

Thanks for the contribution! Before we can merge this, we need @Cr1stal to sign the Salesforce Inc. Contributor License Agreement.