fix(backend): honor launcher S3 override maxRetries

[jeffspahr]

Bug description

The launcher S3 provider override path uses the default provider's maxRetries value when an override supplies its own value.

In backend/src/v2/config/s3.go, the matching override branch checks override.MaxRetries != nil but writes *p.Default.MaxRetries into session parameters. This has two incorrect outcomes:

• an override value is silently ignored in favor of the default value; and
• if the override sets maxRetries while the default omits it, dereferencing p.Default.MaxRetries can panic.

This was found while auditing frontend reconstruction of kfp-launcher provider selection after MLMD removal. It is a backend launcher bug rather than a frontend migration fix, so it should be corrected and validated in the authoritative Go implementation.

Reproduction

Configure an S3 provider with a matching bucket/key-prefix override:

s3:
  default:
    maxRetries: 3
    credentials:
      fromEnv: true
  Overrides:
    - bucketName: team-bucket
      keyPrefix: team-a
      maxRetries: 9
      credentials:
        fromEnv: true

Resolve session information for s3://team-bucket/team-a/model.

Expected: maxRetries is 9.

Actual: maxRetries is 3.

If default.maxRetries is omitted while the override supplies maxRetries, the same path may dereference a nil default pointer.

Acceptance criteria

• A matching override writes override.MaxRetries into session parameters.
• An override can set maxRetries when the default omits it without panicking.
• When the override omits maxRetries, the documented default/provider inheritance behavior is preserved.
• Table-driven tests cover default-only, override-only, differing default/override values, and no configured value.
• Cross-language launcher-provider fixtures used by the frontend reconstruction include this field so future behavior cannot drift silently.

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[Priyanshu-u07]

/assign