Bug 2040381 - Register security event for Access Connector connection error

[jporter-dev]

Description

Bugzilla: Bug-2040381

Part 3 of 3. Adds a security event that records when the Access Connector enters an error state because a site failed to load due to a proxy connection error.

• Registers a new access_connector.proxy_error event metric with the domain as additional metadata, sent in the enterprise ping.
• Since Access Connector is an Enterprise-only feature, the new metric is added in enterprise/metrics.yaml using the Enterprise Products :: Firefox tag.
• Metric is uses data_sensitivity: highly_sensitive since it sends the browsed domain as an extra key.
• Records the event once per error transition. Adds a guard to prevent re-firing on subsequent update calls for the same domain while the page remains in the error state.

Note: this is stacked on top of #1048. Relevant commit is Bug 2040381 - Register telemetry metric for Access Connector proxy error.

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Dependencies / Related Issues

• Depends on: Bug 2040381 - Show error state in Access Connector urlbar button and panel when proxy is unavailable #1048
• Part 3 of: Bug 2040381 - Show custom error page when Access Connector is unavailable #1033

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Testing

• Manual testing performed

Steps to verify changes:

Follow steps outlined in #1048

Expected result:

access_connector.proxy_error event is submitted to console. Verifiable in either the Browser Toolbox network tab or in the console itself.