[Iris] Improve MCP babysitting diagnostics

[dlwh]

Switch MCP worker metadata serialization to protobuf json_format and add Zephyr coordinator thread-liveness to status/diagnosis so zombie coordinators surface in structured output.

Part of #5018.

[claude]

Claude finished @dlwh's task in 4m 10s —— View job

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Code review

No issues found. Checked for bugs and CLAUDE.md / AGENTS.md compliance.

Coverage notes (FYI, not issues):

• _attribute_value_to_json correctly handles all three AttributeValue oneof cases (string_value, int_value, float_value) including the int64 → string round-trip from MessageToDict. Matches old WhichOneof behavior.
• _worker_metadata_to_json's forced int-assignment pattern correctly normalizes int64 string output from MessageToDict and fills zero-defaults that would otherwise be omitted.
• setdefault for string fields correctly fills empty strings when unset while preserving values MessageToDict already emitted.
• Switching to MessageToDict naturally adds tpu_worker_hostnames / tpu_chips_per_host_bounds to the payload when set — previously omitted. Aligns with the stated goal of using protobuf JSON formatting, and the test covers tpu_worker_hostnames.
• The new test exercises large 64-bit values (memory_bytes=123_456_789_012, disk_bytes=987_654_321_000) and all three AttributeValue variants, matching the PR description.