Move setLastReconciledSpec to after successful transition start

[Yanisdje]

This fixes a bug where the last reconciled spec on the flink b/g deployment shows an updated spec even when the update failed.

Summary

• Move setLastReconciledSpec() to after startTransition() and startSavepointRedeployTransition() succeed, so it's not stamped if the transition fails
• Previously, lastReconciledSpec was set before the transition call — if the transition failed or was later aborted, lastReconciledSpec reflected the new spec while the active child still ran the old one. Subsequent deploys would see no diff and fall into PATCH_CHILD (in-place upgrade) instead of a proper B/G transition
• Add assertion in verifySavepointFetchFailureRecovery confirming lastReconciledSpec does not contain the failed spec change

Context

Slack thread: https://shopify.enterprise.slack.com/archives/C0ALAR1TLP8

Test plan

• verifySavepointFetchFailureRecovery passes with new assertion
• Full B/G controller test suite passes (72 tests)

Fixes Shopify/streaming-compute#882

[gyfora]

How do these 2 PRs relate to each other? Should we close one?
#1072
#1076