Alan-Jowett / sonde / 26606283935

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feat(azure-companion): add tracing instrumentation for diagnostics (#1088)

Add structured tracing via the `tracing` crate to sonde-azure-companion.
Previously the companion had no logging - only `eprintln!` for a few
error paths - making it impossible to diagnose runtime issues like
messages not being read from the desired-state queue.

Instrumentation added at key points:
- `info` on startup: effective queue config (endpoint, queue names),
  service principal credentials (tenant/client ID), connector connect,
  and bridge lifecycle (start/shutdown)
- `debug` on message events: queue receive, forward to connector,
  upstream publish, message complete/abandon
- `trace` on each downstream queue poll (fires every ~1s when idle)
- `warn` / `error` on failures: token acquisition, HTTP errors, decode
  errors, connector write failures, bridge runtime exit

Tracing initialization:
- CLI `run` mode: stderr subscriber controlled by `RUST_LOG`
- Windows service mode: file-backed subscriber writing to
  `companion.log` in the state directory (defaults to `info` level
  when `RUST_LOG` is unset, since stderr is invisible for services)
- Uses `try_init()` to avoid panicking on double-init in the Windows
  service dispatch path

Co-authored-by: Alan Jowett <alan.jowett@microsoft.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

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