Pull based capabilities from adapter

[pranavjoshi001]

Changelog Entry

• Added pull-based capabilities system for dynamically discovering adapter capabilities at runtime, in PR #5679, by @pranavjoshi001

Description

Introduces a Capabilities system that enables WebChat to dynamically discover and consume capabilities from chat adapters at runtime.

This allows adapters(directline, ccv2 etc..) to expose configuration such as voice settings without requiring WebChat to have compile-time knowledge of specific adapter implementations. The system follows a pull-based model where adapters expose getter functions and emit events when capabilities change.

Design

Pull-Based Architecture

• Adapters expose getters: Adapters implement getter functions (e.g., getVoiceConfiguration()) to provide capability values
• Event-driven updates: Adapters emit capabilitiesChanged events as a "nudge" to signal WebChat should re-fetch capabilities
• Registry-based discovery: A capability registry maps capability keys to getter function names, making the system extensible

Pure Derivation Pattern

• Uses useReduceMemo to derive capabilities from the activity stream
• A synthetic init marker ensures initial fetch happens on mount
• Capabilities are only re-fetched when:
  1. Component mounts (via init marker)
  2. A capabilitiesChanged event is detected in activities

Reference Stability

• Individual capability objects maintain reference equality when unchanged
• Consumers using selectors only re-render when their specific capability changes
• Uses shallow equality comparison with support for custom comparators

Specific Changes

• Added CapabilitiesComposer component that provides capabilities context
• Added useCapabilities(selector) hook for consuming capabilities with selector pattern
• Added fetchCapabilitiesFromAdapter utility to pull capabilities from adapter getters
• Added capabilityRegistry to map capability keys to adapter getter names
• Added Capabilities and VoiceConfiguration types
• Added valibot schema validation for capabilitiesChanged events
• Exported useCapabilities hook from botframework-webchat/hook
• Added setCapability(getterName, value, options) helper to test emulator
• Added useCapabilities.html test covering initial fetch, event-driven updates, and reference stability
• Added CAPABILITIES.md documentation

• I have added tests and executed them locally
• I have updated CHANGELOG.md
• I have updated documentation

Review Checklist

This section is for contributors to review your work.

• Accessibility reviewed (tab order, content readability, alt text, color contrast)
• Browser and platform compatibilities reviewed
• CSS styles reviewed (minimal rules, no z-index)
• Documents reviewed (docs, samples, live demo)
• Internationalization reviewed (strings, unit formatting)
• package.json and package-lock.json reviewed
• Security reviewed (no data URIs, check for nonce leak)
• Tests reviewed (coverage, legitimacy)