feat(macros): support macros in doc attributes for tool macro

[rliang]

Add support for macros in doc attributes parsed by the tool macro.

Motivation and Context

Currently the #[tool] macro only supports literal strings for #[doc] attributes, and some codebases use external docs, .e.g, #[doc = include_str!(...)].

How Has This Been Tested?

Test cases have been added.

Breaking Changes

N/A

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

[Copilot]

Pull Request Overview

This PR adds support for macros in doc attributes for both the tool and prompt macros, enabling the use of external documentation sources like #[doc = include_str!(...)] instead of just literal strings.

• Changes the description field type from String to Expr in both tool and prompt attribute structs
• Updates the extract_doc_line function to handle macro expressions and preserve them in the generated code
• Adds comprehensive test coverage for the new macro support functionality

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
crates/rmcp-macros/src/tool.rs	Updates tool macro to support macro expressions in doc attributes and adds test for include_str! usage
crates/rmcp-macros/src/prompt.rs	Updates prompt macro to support macro expressions in doc attributes and adds corresponding test case
crates/rmcp-macros/src/common.rs	Modifies extract_doc_line function to handle both literal strings and macro expressions, using concat! for combining multiple doc lines

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