docs(pr-template): add an Assumptions section

Action item from the v1.100.0 OOM RCA. LIT-6780 recorded "running without
--detailed_debug was not tried" as unverified; the fix PR closing it stated
"Only happens with --detailed_debug on" as fact without running that test, and
the untested half is where the customer-facing OOM lived. Nothing in the
template asked for the hedge, so it disappeared between the ticket and review.

The section asks for each untested claim plus what breaks if it is wrong, and
for any hedge on the linked ticket to be carried forward or explicitly closed
out, so reviewers and coding agents have something concrete to attack.

Co-Authored-By: Claude Code <noreply@anthropic.com>

.github/pull_request_template.md

@@ -129,6 +129,23 @@ If you're seeing a delay in your PR being merged, ping the LiteLLM Team on [Slac
      human reader
      Leave this section empty if there are none -->
 
+## Assumptions
+
+<!-- One bullet per claim this PR relies on that you did not actually test, each written as the claim
+     followed by what breaks if it turns out to be wrong. Write it for a reviewer who wants to attack
+     the weakest one, not to reassure them
+     Anything that narrows the scope of a bug belongs here unless you ran the test that proves it:
+     "only reproduces with X on", "no user-observable behavior difference", "this path is debug-only",
+     "no caller passes that shape". A claim you did verify is not an assumption; put the proof in
+     Screenshots / Proof of Fix instead
+     If the linked issue or ticket recorded something as untested or unverified, carry it into this
+     section or say here which run closed it out. Do not drop it silently
+     Example:
+     - The alert path is debug-only, so the growth cannot be hit at default log level. If a second
+       caller stringifies the same structure without a level gate, this ships the bug to every
+       deployment. Not tested: no run with the debug flag off
+     Write "None" if the change rests on nothing untested -->
+
 ## QA runbook
 
 <!-- Only needed when your PR edits tests/e2e; delete this section otherwise