fix: deflake //rs/tests/consensus/tecdsa:tecdsa_key_rotation_test (#9177)

## Root Cause Analysis

In `await_pre_signature_stash_size`
(`rs/tests/consensus/tecdsa/utils/src/lib.rs`), line 1141 had:

```rust
assert_eq!(sizes.len(), subnet.nodes().count());
```

This runs inside a `retry_with_msg!` loop. When
`MetricsFetcher::fetch()` returns metrics, not all nodes may have
started reporting the `execution_pre_signature_stash_size` metric yet.
For example, only 1, 2, or 3 out of 4 nodes might be reporting. The
`assert_eq!` **panics** in this case, killing the test immediately
instead of allowing the retry loop to continue.

This caused 5 out of 7 flaky failures in the last week with errors like:
- `assertion left == right failed: left: 1, right: 4`
- `assertion left == right failed: left: 2, right: 4`
- `assertion left == right failed: left: 3, right: 4`

The remaining 2 failures were timeouts (600s), likely caused by the same
timing sensitivity.

## Fix

Replaced `assert_eq!` with `bail!` so the retry loop continues when not
all nodes have reported the metric yet, giving them time to catch up.

## Verification

All 3 runs passed with `--runs_per_test=3 --jobs=3` (avg 271.5s).

---
This PR was created following the steps in
`.claude/skills/fix-flaky-tests/SKILL.md`.

Author: basvandijk

rs/tests/consensus/tecdsa/utils/src/lib.rs

@@ -1138,7 +1138,13 @@ pub fn await_pre_signature_stash_size(
                     let Some(sizes) = val.get(metric) else {
                         bail!("Metric {metric} not found in {val:?}");
                     };
-                    assert_eq!(sizes.len(), subnet.nodes().count());
+                    if sizes.len() != subnet.nodes().count() {
+                        bail!(
+                            "Metric {metric} only reported by {} out of {} nodes",
+                            sizes.len(),
+                            subnet.nodes().count()
+                        );
+                    }
                     for size in sizes {
                         if *size != expected_size {
                             bail!(