Fix membrane pointerization for structured tool output

[objectiveous]

Summary

Structured tool results were converted with SendableValue.description before being passed into Membrane pointerization. That representation is not a JSON contract: it does not escape ", \, or \n, so any structured tool output containing those characters produces a non-parsable pointer payload.

This changes the Agent tool-output boundary to serialize non-string SendableValue results as canonical JSON (sorted keys, proper escaping, fragments allowed) before Membrane transforms or pointerizes them. Plain string tool results still pass through unchanged.

What changed

• Agent.toolOutputText(for:) — new helper. String passes through; everything else goes through JSONSerialization with [.sortedKeys, .fragmentsAllowed] so the conversation/transcript/pointer payload is parsable JSON.
• NaN/Infinity hardening. JSONSerialization raises an Objective-C NSException on non-finite doubles, which Swift do/catch cannot trap. The helper pre-screens the value tree and falls back to description for non-finite doubles instead of crashing the agent.
• Diagnostic logging. A genuine JSONSerialization failure now logs at Log.agents.warning instead of being silently swallowed.
• MCP SDK test compatibility. SwarmMCPServerServiceTests is updated to the newer CallTool.Result.Content.text(text:annotations:_meta:) case shape so the test target compiles against the currently-pinned MCP SDK. Without this the test target won't build, so we can't deliver the fix without it.

Tests

Local upstream verification (CI runs are currently blocked by an Actions billing issue on the upstream account):

SWARM_HIVE_RUNTIME=1 SWARM_INCLUDE_HIVE=1 swift test --no-parallel
# Test run with 1708 tests in 234 suites passed after 17.297 seconds.

Targeted suites:

• swift test --filter AgentToolOutputTextTests — 8 tests, all pass (new file)
• swift test --filter MembraneIntegrationTests — 7 tests, all pass (includes the new pointerizedStructuredToolOutputResolvesAsJSON integration test)
• swift test --filter SwarmMCPServerServiceTests — 13 tests, all pass

The new unit tests in AgentToolOutputTextTests cover:

• Plain string passthrough (with embedded quotes/newlines preserved verbatim)
• Dictionary serializes as JSON with sorted keys and proper escape sequences
• Nested dictionary/array round-trip via JSONSerialization.jsonObject
• Scalar fragments (Int, Double, Bool, null)
• Top-level array
• Non-finite double fallback at the top level and nested inside a dictionary (regression guard against NSException crash)

Notes for the reviewer

• The PR is rebased on current main (02537a6, including the Wax 0.1.20 fix from Bump Wax pin from 0.1.19 to 0.1.20 (Swift 6.2 strict-concurrency fix) #80).
• The MCP SDK case-pattern update is technically unrelated to the membrane fix but is required for the test target to compile against the current MCP SDK pin — happy to split into a separate PR if preferred, but the fix can't be tested without it.
• Behavior change visible to the model: dictionary/array tool outputs now reach the model as canonical JSON instead of description's JSON-ish format. This is strictly an improvement in correctness; full local suite passes confirm no test snapshots assumed the old representation.