Fix nomad certificate backoff overflow

[peterjan]

This PR fixes an overflow in the backoff algo in the certificate provider. It's hard to reach though, but I found that we fixed a similar issue in 2151fb7 in the connectivity manager so figured we probably want to harden this here as well. Found out through SiaFoundation/indexd#1119 which fixes a similar overflow.

[Copilot]

Copilot review overview

🟢 Approval recommended

The bounded calculation preserves expected behavior and is adequately covered by regression tests.

Review effort: Balanced
Findings: None

What changed in this PR

Fixes certificate retry backoff overflow by safely clamping exponential growth.

Changes:

• Replaces floating-point backoff calculation with bounded bit shifting.
• Adds regression and boundary tests.
• Documents the patch in a changeset.

File	Description
certificates/​providers/​nomad/​nomad.go	Prevents backoff duration overflow.
certificates/​providers/​nomad/​nomad_test.go	Tests backoff limits and edge cases.
.changeset/​fix_certificate_backoff_overflow.md	Records the bug fix.

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