[Enhancement]: Add an opaque annotation channel to tool-execution intercepts

[bbednarski9]

Problem

Relay's tool-execution intercept chain carries only the application-visible Json result. Relay needs an adjacent, optional metadata channel so middleware can annotate a tool execution without changing that result or imposing a result schema on the caller.

Relay-owned shape

pub struct ToolExecutionFrame {
    pub result: Json,
    pub annotation: Option<Json>,
}

pub struct ToolExecutionFrameOutcome {
    pub frame: ToolExecutionFrame,
    pub pending_marks: Vec<PendingMarkSpec>,
}

Add a frame-aware callback and continuation variant to the existing tool-execution registry:

ToolExecutionFrameFn(
    tool_name: &str,
    args: Json,
    next: ToolExecutionFrameNextFn,
) -> Result<ToolExecutionFrameOutcome>

Required semantics

• Raw and frame-aware callbacks use one registry, priority order, and continuation chain.
• The existing raw callback API and application-visible Json result are unchanged.
• annotation is opaque JSON. Relay may transport, sanitize, and export it, but does not define or interpret its schema. JSON null means absent.
• Frame-aware middleware may explicitly preserve, replace, or remove the annotation.
• A raw callback preserves a downstream annotation only when it calls next exactly once and returns a structurally unchanged downstream result. Ambiguous cases drop the annotation.
• Existing continuation, error, short-circuit, and pending-mark behavior remains unchanged.
• The frame-aware variant is additive across Relay's existing in-process, worker, and native plugin surfaces.

Acceptance criteria

• Preserve, replace, and remove work through a mixed raw/frame-aware chain.
• Raw pass-through preserves annotations only in the unambiguous case above.
• Existing raw-result callers remain behaviorally compatible.
• No second registry or execution chain is introduced.

Out of scope

• A canonical tool-result schema.
• Changes to harness-owned tool dispatch, receipt, or result semantics.
• A new service, process, or sidecar.