Fix non-deterministic map iteration in manifest resolver

[Copilot]

The resolveManifestItems function iterated over a secrets map, causing non-deterministic behavior when multiple manifest items mapped to the same environment variable. Map iteration order is randomized in Go, so "last one wins" semantics were unpredictable.

Changes:

• Iterate over manifestItems.Items in order instead of the secrets map
• Removed now-unnecessary envVarSecretMap intermediary
• Updated test to verify deterministic "last one wins" behavior

Before:

for key, value := range secrets {
    if envVar, ok := envVarSecretMap[key]; ok {
        result.AppendItem(envVar, value)
    }
}

After:

for _, item := range manifestItems.Items {
    if value, ok := secrets[item.Name]; ok {
        result.AppendItem(item.Env, value)
    }
}

When multiple manifest entries target the same environment variable, the last entry in the manifest now consistently wins.

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