Fixed the same missing Err() issue in context.go

[IDisposable]

Since #763 was merged instead of #758, only event.go was fixed. Need the same fix for context.go

[Copilot]

Pull request overview

Fixes a regression where Context.Err() could drop the main error field when ErrorStackMarshaler(err) returns nil, aligning context.go behavior with the earlier fix applied to event.go (PR #763).

Changes:

• Update Context.Err() to not return early when ErrorStackMarshaler returns nil, so AnErr still records the error field.
• Update TestContext_ErrWithNilStackMarshaler to assert the error field is still emitted when stack marshaling yields nil.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
context.go	Removes the early return on nil stack-marshaler output so the main error field is still logged.
context_test.go	Adjusts expectations/documentation to verify error logging is preserved even without a stack trace.

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[Copilot]

The inline comment says “Fall through” inside a Go type-switch, but there is no fallthrough statement here. This wording can be misleading since “fallthrough” has a specific meaning in Go switches; consider rephrasing to something like “do nothing here; continue after the switch and still log the error via AnErr below.”

Suggested change

	// attach. Fall through and still log the error via AnErr below.
	// attach. Do nothing here; continue after the switch and still log
	// the error via AnErr below.

[IDisposable]

I copied the language from the other PR's change in event.go. Consistency rules (or I could change both in this PR, what say you @rs)?