Add handling of missing tool calls/tool call responses

[akavi]

What does this PR do?

Bug report from Balto

1. Updates LlmProvider to correctly handle unpaired tool_call/tool_call_response
2. Moved the other validation/normalization from LlmAgent -> LlmProvider. Feels like the more natural place for it.
   a) LlmAgent is about maintaining the state of the agent, LlmProvider is about wrapping the Llm conversation API in a consistent interface

Based off of #159 since that does a bunch of rework of the LlmProvider code, and I'd rather not deal with the rebase mess.

Type of change

• Bug fix
• New feature
• Breaking change
• Documentation
• Other: ___________

Testing

Unit tests

Checklist

• I have read the contributing guidelines
• I have added tests that prove my fix is effective or that my feature works
• I have formatted my code with make format

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Note

Medium Risk
Changes the pre-flight normalization for every LLM chat call (message filtering, terminal validation, and tool-call pairing), which could alter conversation history and tool execution in edge cases despite being defensive.

Overview
Centralizes message validation in LlmProvider by adding _normalize_messages and applying it in LlmProvider.chat, returning an empty stream when the conversation is empty/invalid.

Normalization now filters empty user/assistant messages, removes unpaired tool_calls and orphaned tool responses, and enforces terminal-message rules (can’t end on assistant; last user message must be non-empty), replacing the earlier skip logic previously done in LlmAgent.

LlmAgent no longer pre-validates message sequences before calling the provider, and _build_messages now preserves empty UserTextSent/AgentTextSent contents (as "") for the provider to handle.

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