V2: Don't trigger handler signal on success

[srikrsna-buf]

Don't trigger handler signal on success. Implements the behavior highlighted in #1117 (comment).

This is a breaking change. In v1 the signal will always trigger on completion.

[timostamm]

Can we update the JSDoc on HandlerContext? E.g.

  /**
   * An AbortSignal that triggers when the deadline is reached, or when an
   * error occurs that aborts processing of the request.
   *
   * The signal can be used to automatically cancel downstream calls.
   */
  readonly signal: AbortSignal;