feat(server): allow tool calls outside passthrough mode without dialog flows

[christinaexyou]

… flows

Description

Tool calls were previously gated on passthrough mode. They are now allowed
in any config as long as no dialog flows are defined (user_messages or
single_call dialog rails). Streaming requests still reject tools/tool_choice/
parallel_tool_calls. We explicitly reject tool call requests if dialog flows or streaming is enabled in the config.

Related Issue(s)

• Addresses feature: OpenAI tools param support for non-passthrough guardrails configs #2057

Verification

AI Assistance

• No AI tools were used.
• AI tools were used; a human reviewed and can explain every change (tool: ___).

Checklist

• I've read the CONTRIBUTING guidelines.
• This PR links to a triaged issue assigned to me.
• My PR title follows the project commit convention.
• I've updated the documentation if applicable.
• I've added tests if applicable.
• I've noted any verification beyond CI and any checks I couldn't run.
• I did not update generated changelog files manually.
• I addressed all CodeRabbit, Greptile, and other review comments, or replied with why no change is needed.
• @mentions of the person or team responsible for reviewing proposed changes.

[github-actions]

PR merge guidance

@christinaexyou thanks for the PR. GitHub is currently blocking merge for one or more repository requirements:

• 2 commits do not have a verified signature (ee0bf26, 9c8f260). Please sign the commits and force-push the updated branch.

Relevant guide:

• Signed commits: https://github.com/NVIDIA-NeMo/Guardrails/blob/develop/CONTRIBUTING.md#commit-signing
• Contribution guide: https://github.com/NVIDIA-NeMo/Guardrails/blob/develop/CONTRIBUTING.md