fix(rmcp): flatten Resource variant of PromptMessageContent

[ynishi]

Fixes #842

Motivation and Context

PromptMessageContent::Resource currently serializes with a doubly-nested
resource object, producing:

{
  "type": "resource",
  "resource": {
    "resource": { "uri": "file:///example.md", "text": "...", "mimeType": "text/markdown" }
  }
}

The MCP specification (Prompts — Embedded Resources) defines the wire shape
as a single, flat resource field:

{
  "type": "resource",
  "resource": { "uri": "file:///example.md", "text": "...", "mimeType": "text/markdown" }
}

The other variants of PromptMessageContent (Text, Image, ResourceLink)
already use #[serde(flatten)] and serialize flat. Resource is the only
outlier: the resource: EmbeddedResource field was missing #[serde(flatten)],
so the outer enum tag wrapper plus the inner struct's own resource field both
serialize, yielding the double nesting.

This breaks interop with any MCP client that follows the spec literally — the
inner {uri, text, mimeType} is unreachable without descending two levels.

How Has This Been Tested?

• Added a regression test (test_prompt_message_resource_serialization_is_flat)
  that constructs a PromptMessage::new_resource(...), serializes it, and
  asserts:
  • content.type == "resource"
  • content.resource.uri, content.resource.mimeType, content.resource.text
    are reachable directly (flat shape).
  • content.resource.resource is absent (regression guard against the
    doubly-nested shape).
• Regenerated the affected schema snapshot via UPDATE_SCHEMA=1.
• cargo test --all-features passes locally for the rmcp crate (lib + all
  integration tests).
• cargo +nightly fmt --all -- --check and
  cargo clippy --all-targets --all-features -- -D warnings pass.

Breaking Changes

Wire format: yes, but in the spec-compliance direction. Any client that
relied on the previous (non-spec) doubly-nested shape would break, but such
clients are already non-conformant against the MCP spec and against the other
content variants in this same enum.

Rust API: none. The variant's field name (resource) and type
(EmbeddedResource) are unchanged; only the serde representation is corrected.
cargo semver-checks should be unaffected.

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 not work as expected)

Additional Context

Single-line behavioral fix in crates/rmcp/src/model/prompt.rs: add
#[serde(flatten)] to the Resource variant's inner field, matching the
pattern used by Image and ResourceLink in the same enum.