fix: correct failed requests delta count in periodic log message

[Copilot]

getOperationMode() mutates previousState before the ERROR message computes the delta, making this.stats.state.requestsFailed - previousState.requestsFailed always 0. The || this.stats.state.requestsFailed fallback then prints the total failed count instead of the interval delta.

Changes

• getPeriodicLogger — changed getOperationMode() return type from 'ERROR' | 'REGULAR' to { mode, failedDelta }, exposing the delta already computed before previousState is overwritten
• Removed the broken ... - previousState.requestsFailed || this.stats.state.requestsFailed expression; message now uses failedDelta directly

// Before — previousState already updated when this runs, delta always 0
message = `Experiencing problems, ${
    this.stats.state.requestsFailed - previousState.requestsFailed || this.stats.state.requestsFailed
} failed requests in the past ${this.statusMessageLoggingInterval} seconds.`;

// After — delta captured inside getOperationMode() before previousState is mutated
const { mode: operationMode, failedDelta } = getOperationMode();
// ...
message = `Experiencing problems, ${failedDelta} failed requests in the past ${this.statusMessageLoggingInterval} seconds.`;

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Closes #3483

[barjin]

Looking at this, I suppose we always logged the wrong number here (previousState would always be equal to this.stats.state), so the message would always log this.stats.state.requestsFailed 👀