Fix browser.newPageInContext bug

[inancgumus]

What?

Fixes a bug when newPageInContext is called without a BrowserContext. Instead of panicking, we now try to use the default browser context if there isn't a current BC, and return an error (as before) if the requested browser ID and the BCs don't match. So, we preserve the current behavior and turn panics into errors.

This fix doesn't solve the underlying race condition issue, though. The next step could be to avoid race conditions between BrowserContext.Close and BrowserContext.NewPage (added #5328).

Why?

When newPageInContext is called without a BrowserContext, we attempt to log with a missing BrowserContext.id, resulting in nil pointer exceptions:

panic: runtime error: invalid memory address or nil pointer dereference [recovered, repanicked]
[signal SIGSEGV: segmentation violation code=0x1 addr=0x58 pc=0x12d0f11]

goroutine 1055774 [running]:
go.opentelemetry.io/otel/sdk/trace.(*recordingSpan).End.deferwrap1()
go.opentelemetry.io/otel/sdk@v1.37.0/trace/span.go:467 +0x25
go.opentelemetry.io/otel/sdk/trace.(*recordingSpan).End(0xc02cda4d20, {0x0, 0x0, 0xc00023ac40?})
go.opentelemetry.io/otel/sdk@v1.37.0/trace/span.go:506 +0xb79
panic({0x1afe4c0?, 0x319c9a0?})
runtime/panic.go:783 +0x132
go.k6.io/k6/internal/js/modules/k6/browser/common.(*Browser).newPageInContext(0xc0002a6000, {0xc00094e280, 0x20})
go.k6.io/k6/internal/js/modules/k6/browser/common/browser.go:457 +0x2f1
go.k6.io/k6/internal/js/modules/k6/browser/common.(*BrowserContext).NewPage(0xc0009bf080)
go.k6.io/k6/internal/js/modules/k6/browser/common/browser_context.go:267 +0x145
go.k6.io/k6/internal/js/modules/k6/browser/browser.mapBrowserContext.func14.1()
go.k6.io/k6/internal/js/modules/k6/browser/browser/browser_context_mapping.go:184 +0x1c
go.k6.io/k6/internal/js/modules/k6/browser/k6ext.promise.func1()
go.k6.io/k6/internal/js/modules/k6/browser/k6ext/promise.go:24 +0x2c
created by go.k6.io/k6/internal/js/modules/k6/browser/k6ext.promise in goroutine 72
go.k6.io/k6/internal/js/modules/k6/browser/k6ext/promise.go:23 +0x9a

Checklist

• I have performed a self-review of my code.
• I have commented on my code, particularly in hard-to-understand areas.
• I have added tests for my changes.
• I have run linter and tests locally (make check) and all pass.

Checklist: Documentation (only for k6 maintainers and if relevant)

Please do not merge this PR until the following items are filled out.

• I have added the correct milestone and labels to the PR.
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