Preserve internal dispatch segment timings on worker operation logs

[anth-volk]

Internal Flask dispatch requests currently create their own request timing dictionary. Because those internal request logs are intentionally suppressed, segment timings recorded inside the dispatched endpoint can be absent from the visible parent worker operation log.

Acceptance criteria:

• Internal dispatch requests running inside a parent operation share the parent operation timing data.
• Ordinary nested/non-internal request contexts keep isolated timing data.
• The behavior is covered by runtime tests.
• The fix includes a changelog fragment.