fix(observability): prevent "Converting circular structure to JSON" crash from circular cy.task payloads [SDK-6016]

[harshit-browserstack]

Problem

SDK-6016 When BrowserStack Test Observability is enabled, the Cypress instrumentation captures command args (command.attributes.args) and cy.log items raw and ships them to the Node plugin via cy.task(...), which JSON-serializes the payload (browser → Node). If a command/log arg is a circular Cypress runtime object — e.g. one whose renderOptions.host references itself — serialization throws:

Converting circular structure to JSON
    --> starting at object with constructor 't'
    |     property 'renderOptions' -> object with constructor 'Object'
    --- property 'host' closes the circle

…and aborts the entire test run. There was no try/catch around the flush, so an o11y serialization failure breaks the user's tests — contrary to the graceful-degradation contract.

This commonly surfaces in component-testing setups (@cypress/vite-dev-server / @cypress/vue) where mount/dev-server config objects are self-referential; e2e args are usually primitive and never trip it, which is why it stayed latent.

Fix

• Add a WeakSet-based getCircularReplacer() and a sanitizeForTask() that decycles the payload (circular refs → "[Circular]") before it is handed to cy.task.
• Apply it at both flush points (beforeEach/afterEach) and at the command:enqueued log stringify.
• sanitizeForTask wraps serialization in a try/catch, so o11y instrumentation can never break the user's test run.

Non-circular fields are preserved; only true cycles are replaced.

Verification

• Local (byte-faithful): a self-referential renderOptions.host object throws the exact error under raw JSON.stringify; with sanitizeForTask it ships cleanly.
• Real BrowserStack build: a spec issuing a circular command arg (cfg.renderOptions.host = cfg) with testObservability: true completed 1/1 passed with no "Converting circular structure" in logs — build 0e384b2ee0de66beb4fe0254a85d1c66e937e02b.

Scope

Fix-only — single file (bin/testObservability/cypress/index.js), no version bump, no CHANGELOG (owned by the release PR).

🤖 Generated with Claude Code