Python: support MCP skills of mcp-resource-template type

[vaibhav-patel]

Summary

Adds support for the mcp-resource-template skill kind to the MCP skills source,
addressing #6118.

Per the SEP-2640 binding of Agent Skills Discovery v0.2.0, an mcp-resource-template
entry in skill://index.json omits name and carries an RFC 6570 URI template in
url (e.g. skill://docs/{product}/SKILL.md) that resolves to a SKILL.md resource
once its variables are bound. One template represents a family of skills — one per
variable binding. (Mirrors the .NET McpSkillIndexEntry contract in
Microsoft.Agents.AI.Mcp.)

Changes

• New MCPSkillResourceTemplate (exported, experimental MCP_SKILLS):
  • variables — template variable names (RFC 6570 Level-1).
  • expand(variables) — simple string expansion; values percent-encoded so they can't
    inject path segments/schemes; errors on missing variables.
  • materialize(*, name, variables, description=None) — returns a concrete MCPSkill
    reusing existing fetch machinery.
• MCPSkillsSource.get_resource_templates() — surfaces template entries separately
  from get_skills(). Concrete skills are not auto-materialized at discovery because the
  variable values aren't in the index; callers bind them explicitly. get_skills()
  behavior is unchanged.
• Tests: 17 new covering variable extraction, expansion/encoding/error paths,
  materialize, discovery, and end-to-end resolution.

Open design question (for maintainers)

Auto-enumerating concrete skills during discovery requires a source of variable values
(host/user config, or resources/templates/list + completion). That decision is left open
intentionally; this PR provides the parsing, validation, and explicit-materialization
primitives. Happy to extend with an auto-materialization strategy once the sourcing
approach is decided. This also keeps Python aligned with .NET #6117, which currently skips
templates as well.

Verification

• uv run pytest packages/core/tests/core/test_mcp_skills.py → 68 passed.
• ruff check / ruff format --check clean on changed files.

Addresses #6118.

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[Copilot]

Pull request overview

Adds Python support for MCP skill index entries of type mcp-resource-template (RFC 6570 URI templates) by introducing a resource-template abstraction and a parallel discovery path that surfaces templates separately from concrete skills.

Changes:

• Introduces MCPSkillResourceTemplate with variable extraction, safe URI expansion (percent-encoding), and explicit materialize() to produce concrete MCPSkill instances.
• Extends MCPSkillsSource with get_resource_templates() to discover template entries without auto-materializing skills.
• Adds unit tests covering template parsing/expansion/materialization and discovery behavior, and exports the new type from agent_framework.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
python/packages/core/agent_framework/_skills.py	Adds MCPSkillResourceTemplate and MCPSkillsSource.get_resource_templates() to support discovery/materialization of mcp-resource-template entries.
python/packages/core/agent_framework/init.py	Exports MCPSkillResourceTemplate from the package surface (__all__ + imports).
python/packages/core/tests/core/test_mcp_skills.py	Adds tests for template behavior and for discovering templates separately from concrete skills.