fix(lambda): return X-Amz-Log-Result for buffered Invoke with LogType=Tail

[vieiralucas]

Summary (1.20)

A synchronous Invoke with LogType=Tail must return the base64 of the last 4 KiB of the invocation's logs in X-Amz-Log-Result (the SDK's LogResult). The buffered RequestResponse path never set it — only the streaming eventstream path carried the field, and even there it was always empty. So aws lambda invoke --log-type Tail and test harnesses got nothing.

Adds a per-instance log-capture hook to the runtime backend trait (instance_logs): the docker/podman backend tails docker logs, the k8s backend fetches the Pod's logs via the kube API. invoke_inner optionally captures the instance's log tail while the slot is still held; the buffered handler, when LogType=Tail, base64s the last 4 KiB into X-Amz-Log-Result. The other ~22 invoke callers are unchanged (they go through the no-capture wrapper).

Non-code surface

No new API surface — fills in a documented response header. No SDK/docs/metadata change applies (checked).

Test plan

• E2E (Docker-gated) invoke_with_log_type_tail_returns_log_result: a function prints a marker; Invoke with LogType=Tail returns a LogResult decoding to contain it.
• cargo test -p fakecloud-lambda --lib (207) passes; clippy clean.

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Summary by cubic

Fixes buffered Lambda Invoke so X-Amz-Log-Result is returned for LogType=Tail, base64-encoding the last 4 KiB of the function logs. Works for both Docker and Kubernetes backends via per-instance log capture.

• Bug Fixes
  • Return X-Amz-Log-Result for synchronous Invoke with LogType=Tail (last 4 KiB, base64).
  • Add instance_logs backend hook; Docker tails docker logs, K8s reads Pod logs via the kube API.
  • Add E2E test verifying the log tail and register it in the Python runtime partition filter.

^{Written for commit b2e9b6c. Summary will update on new commits.}