Forward errorMessage in WebExtensionApiRequest telemetry

[amitjoshi438]

Summary

• sendAPITelemetry discards errorMessage today: the parameter is accepted and used to build a traceError invocation, but it never lands in properties, so every WebExtensionApiRequestFailure event in Kusto is missing the underlying error detail (status text, response body, OData error code).
• The save flow already builds a useful message via createHttpResponseError / formatHttpErrorMessage (with the truncated Dataverse response body) and forwards it through sendAPIFailureTelemetry -> sendAPITelemetry. This PR plumbs that string into the telemetry payload.
• Adds errorMessage?: string to IWebExtensionAPITelemetryData.properties. sendAPITelemetry now sets the field only when present (so success-path events stay clean) and applies a defensive 500-char cap for non-HTTP callers that may pass a raw unbounded error.message.

Test plan

• npm test (unit, blast-radius check) - 95/95 passing locally
• npm run test-web-integration - blocked locally by VS Code Insiders editor lock; CI should run cleanly
• Existing telemetry test sendAPITelemetry_whenErrorMessageIsPassed_shouldCallSendTelemetryException updated to assert errorMessage is now in properties
• Existing telemetry test sendAPITelemetry_whenErrorMessageNotPassed_shouldCallSendTelemetryException unchanged (validates errorMessage is omitted when not provided)
• After deploy, query Kusto: WebExtensionApiRequestFailure events should carry non-empty errorMessage containing the truncated response body for HTTP failures
• After deploy, run the OData error-code bucketing query above to size the dominant save-400 cause