fix(registry): fix JIT error scoping race

[LukeAVanDrie]

What type of PR is this?

/kind bug

What this PR does / why we need it:

This PR fixes a race condition where concurrent requests waiting for JIT provisioning could incorrectly proceed with an uninitialized flow if the primary requester failed.

The Bug:

1. Request A (Winner) calls WithConnection and starts fr.ensureFlowInfrastructure.
2. Request B (Waiter) blocks on state.initialized.Do.
3. Request A fails (e.g., dynamic band creation fails). jitErr (local var) is set. WithConnection returns error.
4. Request B wakes up. state.initialized.Do returns (no-op).
5. Request B checks its own local jitErr, which is nil because the error was scoped to Request A's stack.
6. Request B proceeds to use the broken flow state, panic or error ensues.

The Fix:

• Introduced initErr field to flowState.
• The result of fr.ensureFlowInfrastructure is now stored on the struct itself.
• All waiters (Winner and Waiters) check state.initErr after the Do block returns.

Testing:

• Added TestFlowRegistry_JITErrorScopingwhich deliberately stalls concurrent requests during JIT provisioning and forces a failure.
• Verified that all concurrent requests now correctly receive the error (previously, N-1 requests would succeed incorrectly).

Which issue(s) this PR fixes:

Followup to @aishukamal's comment here. Thanks for catching this!

Does this PR introduce a user-facing change?:

NONE

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[LukeAVanDrie]

/assign @aishukamal

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In response to this:

/assign @aishukamal

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[ahg-g]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ahg-g, LukeAVanDrie

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[ahg-g]

/lgtm