1e74ca99e9400c251d274661f1fdeeed0dbda10c  [WebDriver BiDi] Implement browsingContext.captureScreenshot command plumbing

Classified as not security-relevant.
Triage note: Implements browsingContext.captureScreenshot with parameter validation and adds the protocol JSON; also corrects a BiDi error-code string. New automation feature plus input validation, not a fix to an existing memory-safety or logic bug.
This commit was flagged by the signal scorer and reviewed by AI (not a human). It is listed only so the triage is auditable; there is no root-cause analysis because this is not a vulnerability fix.

Structural signals: smartptr_member_added (+6.47), null_guard (+2.59), msg_soft (+1.0)