feat(rust): support binary tool results

[cschleiden]

Rust tool results need to preserve binary payloads from MCP tool calls so callers can return images and binary resources to the LLM without collapsing structured results into text.

This updates the Rust SDK coverage around binary tool results by ensuring resource blobs default to application/octet-stream when mimeType is missing or empty, and by adding focused tests for the public binaryResultsForLlm wire shape plus MCP image, resource blob, mixed text/binary, and no-binary conversion behavior.

Validation:

• cd rust && cargo test --all-features
• cargo +nightly-2026-04-14 fmt --check
• cargo clippy --all-features --all-targets -- -D warnings

[Copilot]

Pull request overview

This PR enhances the Rust SDK’s handling of MCP tool results that include binary payloads, ensuring binary resources are preserved for LLM consumption (instead of being forced into text-only results).

Changes:

• Default MCP resource.blob mimeType to application/octet-stream when mimeType is missing or an empty string.
• Add Rust type-level serialization tests to confirm the binaryResultsForLlm wire shape is emitted correctly (and omitted when None).
• Expand MCP conversion tests to cover image content, resource blob content, mixed text/binary, and no-binary cases.

Show a summary per file

File	Description
rust/src/types.rs	Adds focused serialization tests for ToolResultExpanded.binaryResultsForLlm wire output.
rust/src/tool.rs	Treats empty mimeType as missing (defaults to application/octet-stream) and adds targeted MCP conversion tests for binary content.

Copilot's findings

• Files reviewed: 2/2 changed files
• Comments generated: 0

[stephentoub]

Thanks, @cschleiden. I think the Python and Node.js SDKs have the same bug. Would you mind fixing those in this PR as well?