plugins/bundle: prevent ns-level polling by validating intervals

[jjhwan-h]

Fix polling interval validation in Reconfigure to avoid nanosecond ticker loops

Fixes: #8081

Why the changes in this PR are needed?

When Reconfigure() is called on the bundle plugin, the provided bundle configuration
is applied without validating polling.min_delay_seconds and polling.max_delay_seconds.
This means that even when a user explicitly configures a valid value such as:

"min_delay_seconds": 10

the actual polling interval used by the downloader may still fall back to 10 nanoseconds
(or effectively 0 nanoseconds) instead of 10 seconds.
In practice, this results in an extremely tight polling loop and thousands of HTTP requests
per second.

This behavior is incorrect and inconsistent with the user’s configuration, and validation
is required to ensure that the polling delays reflect the intended seconds-based values.

What are the changes in this PR?

• Add validation for polling interval fields during the Reconfigure() flow.

Notes to assist PR review:

• After this fix, the configuration is validated before applying changes, preventing the
  downloader from entering a tight polling loop.
• Changes are limited to v1/plugins/bundle/plugin.go.

Further comments:

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[sspaink]

Thank you @jjhwan-h for fixing this! definitely a bug 🐛

The test TestUpgradeLegacyBundleToMuiltiBundleSameBundle is now failing because it gets further then before. It tries to stop the downloader that was configured incorrectly without a trigger causing a nil de-reference when trying to stop the downloaders.

Let me know if you need any help fixing this test!