docs(model): update model provider documentation to match the new interface

[mkmeral]

Update Custom Model Provider Documentation for Stream-First Interface

Description

• Updated Strands Agents SDK version
• Updated the custom model provider documentation to reflect the current stream-first model interface. Rewrote the implementation guide to focus on the single stream method entry point, clarified that helper methods are optional recommendations rather than requirements, and removed references to historical interface changes to maintain focus on current functionality.

See related feature PR in SDK: strands-agents/sdk-python#400

Type of Change

Content update/revision

Motivation and Context

The documentation was referencing an outdated model interface pattern and needed to be updated to match the current SDK implementation. The previous documentation referenced abstract methods that are no longer required.

This update ensures developers have accurate, current guidance for implementing custom model providers.

Areas Affected

• /docs/user-guide/concepts/model-providers/custom_model_provider.md
  • Section 2: Complete rewrite of stream method implementation
  • Section 4: Updated structured output implementation
  • Section 5: Simplified usage examples
  • Key Implementation Considerations: Updated all subsections to reflect current interface

Screenshots

N/A - Text-only documentation changes

Checklist

• I have read the CONTRIBUTING document
• My changes follow the project's documentation style
• I have tested the documentation locally using mkdocs serve
• Links in the documentation are valid and working
• Images/diagrams are properly sized and formatted
• All new and existing tests pass

Additional Notes

None

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