Add tag probabilities to AI Guard evaluation result

[y9v]

What does this PR do?
This PR adds AIGuard::Evaluation::Result#tag_probabilities attribute, exposing the tag_probs field from the AI Guard API response.

Motivation:
We want to add evaluation result tag probabilities to our SDK, and attach tag probabilities to metastruct.

Change log entry
Yes. AI Guard evaluation results now include tag probabilities from the API response.

Additional Notes:
APPSEC-61897

How to test the change?
CI and manual testing

[github-actions]

Typing analysis

Note: Ignored files are excluded from the next sections.

steep:ignore comments

This PR introduces 1 steep:ignore comment, and clears 1 steep:ignore comment.

steep:ignore comments (+1-1)

❌ Introduced:

lib/datadog/ai_guard/evaluation.rb:65

✅ Cleared:

lib/datadog/ai_guard/evaluation.rb:64

Untyped methods

This PR introduces 1 partially typed method, and clears 1 partially typed method.

Partially typed methods (+1-1)

❌ Introduced:

sig/datadog/ai_guard/evaluation/result.rbs:16
└── def initialize: (::Hash[::String, untyped] raw_response_body) -> void

✅ Cleared:

sig/datadog/ai_guard/evaluation/result.rbs:15
└── def initialize: (::Hash[::String, untyped] raw_response_body) -> void

If you believe a method or an attribute is rightfully untyped or partially typed, you can add # untyped:accept on the line before the definition to remove it from the stats.

[datadog-datadog-prod-us1]

✅ Tests

🎉 All green!

❄️ No new flaky tests detected
🧪 All tests passed

🎯 Code Coverage (details)
• Patch Coverage: 93.75%
• Overall Coverage: 95.35% (-0.02%)

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