Preserve MCP trust settings error sources

[beiwei30]

Summary

• preserve the typed ConfigError source inside the existing McpError::Io boundary when MCP trust settings persistence fails
• keep the established display text and McpError protocol code stable while proving settings-content and token canaries remain redacted
• add focused source-chain and serialized app-protocol regression coverage

Testing

• cargo test -p orbcode-mcp set_server_trust
• cargo test -p orbcode-app-server mcp_set_trust_settings_error_redacts_protocol_response
• cargo test -p orbcode-app-server-client
• cargo test -p orbcode-app-server-transport
• cargo test -p orbcode-tools
• cargo test -p orbcode-mcp
• cargo fmt --all --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo check --workspace
• cargo test --workspace
• scripts/check-docs.sh
• scripts/audit-public-surface.sh
• scripts/audit-brand.sh
• git diff --check

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