feat!: support MCP schema 2026-07-28 and make it the default (v2.0.0)

[hashemix]

📌 Summary

Adds support for MCP schema version 2026-07-28, promotes it to the default
(and latest) version, and refreshes the draft schema, examples, and docs to
match.
This is a breaking change (v2.0.0): the default feature set and crate-root re-exports now resolve to 2026_07_28, and several legacy helper methods have been deprecated/removed.

✨ Changes Made

• Add generated 2026_07_28 schema (mcp_schema.rs, schema_utils.rs,
  validators.rs) plus its Cargo feature
• Promote 2026_07_28 to default and latest, make 2025_11_25 opt-in
• Add ProtocolVersion::V2026_07_28, update latest(), Display, TryFrom
• Updated README + examples to use CallToolRequest/CallToolResult and the
  new delegate-enum API
• Deprecate type_name()/method_name() in favor of *_value()
• Remove ContentBlock::embedded_text_resource/embedded_blob_resource
• Update clippy/test scripts for 2026_07_28 and the deprecated lint
• Add tests/v2026_07_28 integration tests, update docs.rs

🛠️ Testing Steps

📌 Summary

Adds support for MCP schema version 2026-07-28, promotes it to the default
(and latest) version, and refreshes the draft schema, examples, and docs to
match. Breaking change targeting v2.0.0.

🔍 Related Issues

• Fixes #

✨ Changes Made

• Add generated 2026_07_28 schema (mcp_schema.rs, schema_utils.rs,
  validators.rs) plus its Cargo feature
• Promote 2026_07_28 to default and latest; make 2025_11_25 opt-in
• Add ProtocolVersion::V2026_07_28; update latest(), Display, TryFrom
• Update draft schema to the 2026-07-28 contents
• Rewrite README + examples to use CallToolRequest/CallToolResult and the
  new delegate-enum API
• Deprecate type_name()/method_name() in favor of *_value()
• Remove ContentBlock::embedded_text_resource/embedded_blob_resource
• Update clippy/test scripts for 2026_07_28 and the deprecated lint
• Add tests/v2026_07_28 integration tests; update docs.rs/playground features

🛠️ Testing Steps

• ./scripts/run_test.sh and ./scripts/run_clippy.sh
• Run mcp_client_handle_message / mcp_server_handle_message examples

💡 Additional Notes

• Breaking change; release as v2.0.0.