retain valid certs on fetch failures

[deveshdama]

Retain existing valid certificates when new fetch attempts fail, improving service reliability during CA outages. Implements backoff scheduling that respects certificate expiry times.
This PR addresses istio issue#56452

[istio-policy-bot]

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[istio-testing]

Hi @deveshdama. Thanks for your PR.

I'm waiting for a istio member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

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[keithmattix]

/ok-to-test

[deveshdama]

@howardjohn can you please take a look.

[jaellio]

@keithmattix or @Stevenjin8 could you PTAL?

[Sridharpage]

I haven’t seen any updates on this PR for over a month and a half. Could someone please take a look and help move it forward?

[Stevenjin8]

@deveshdama

[deveshdama]

Sorry for the delay.
@jaellio @Stevenjin8 PTAL.

[Stevenjin8]

@deveshdama lgtm, but need a maintainer to review

[deveshdama]

@keithmattix can you please help out with review?

[keithmattix]

I'm not an approved; tagging @istio/wg-networking-maintainers-ztunnel for review