Fix exponential backoff (10secs, 4xx accepted)

[sarroutbi]

This patch introduces two key refinements to the agent's exponential
backoff and retry logic, making it more patient and aligned with
standard network practices.

• Increased Default Backoff Timings (config/base.rs)
  The default values for the exponential backoff have been
  significantly increased:

  • Initial Delay: Changed from 2 seconds to 10 seconds. This makes
    the agent wait longer before the first retry, which is more suitable for
    services that might be slow to initialize.

  • Maximum Delay: Changed from 60 seconds to 300 seconds (5
    minutes). This allows the delay between retries to grow larger,
    accommodating longer-term service disruptions.

• Smarter Retry Strategy (resilient_client.rs)
  The core logic in the custom StopOnSuccessStrategy has been improved
  to be more intelligent about when to retry.

  • Before: The strategy would retry on any non-success status code,
    including 4xx client errors (like 404 Not Found), which are typically
    not temporary.

  • After: The strategy now delegates the decision for non-success
    codes to reqwest-retry's built-in default_on_request_success function.
    This default logic is smarter:

• It will retry on 5xx server errors (e.g., 503 Service Unavailable).

• It will NOT retry on most 4xx client errors (e.g., 404 Not
  Found, 403 Forbidden), as these indicate a problem with the request
  itself, not a temporary server issue.

• Similarly, the handling of network errors is now delegated to
  default_on_request_failure, ensuring consistent and robust behavior.

[ansasaki]

LGTM!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 57.84%. Comparing base (7c892c8) to head (43cdac6).

Additional details and impacted files

Flag	Coverage Δ
e2e-testsuite	57.84% <100.00%> (ø)
upstream-unit-tests	57.84% <100.00%> (ø)

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Files with missing lines	Coverage Δ
keylime/src/config/base.rs	86.90% <ø> (ø)
keylime/src/resilient_client.rs	48.68% <100.00%> (ø)

... and 2 files with indirect coverage changes

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