chore: release v0.3.0

[github-actions]

🤖 New release

• rmcp-macros: 0.2.1 -> 0.3.0
• rmcp: 0.2.1 -> 0.3.0 (⚠ API breaking changes)

⚠ rmcp breaking changes

--- failure type_allows_fewer_generic_type_params: type now allows fewer generic type parameters ---

Description:
A type now allows fewer generic type parameters than it used to. Uses of this type that supplied all previously-supported generic types will be broken.
        ref: https://doc.rust-lang.org/cargo/reference/semver.html#trait-new-parameter-no-default
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.41.0/src/lints/type_allows_fewer_generic_type_params.ron

Failed in:
  Enum ServerInitializeError allows 1 -> 0 generic types in /tmp/.tmpth9RlO/rust-sdk/crates/rmcp/src/service/server.rs:41
  Enum ClientInitializeError allows 1 -> 0 generic types in /tmp/.tmpth9RlO/rust-sdk/crates/rmcp/src/service/client.rs:28

Changelog

rmcp-macros

0.3.0 - 2025-07-15

Added

• unified error type (#308)

Other

• (deps) update darling requirement from 0.20 to 0.21 (#318)

rmcp

0.3.0 - 2025-07-15

Added

• unified error type (#308)
• (transport) add builder & expose child stderr (#305)

Other

• (deps) update schemars requirement from 0.8 to 1.0 (#258)
• (rmcp) bump rmcp-macros version to match (#311)
• fix packages used for server example (#309)

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This PR was generated with release-plz.

[alexhancock]

Thoughts on the breaking change notification @4t145?

Seems minimal impact to consumers but wanted to get your take before releasing