fix(agents): honor MODEL_PROVIDER for subagent routing

[Plaidmustache]

Description

Fixes subagent routing so Task/subagent execution honors configured MODEL_PROVIDER instead of defaulting to Azure.

Type of Change

• Bug fix
• New feature
• Documentation update
• Performance improvement
• Refactoring

Changes Made

• Updated subagent provider resolution in src/agents/executor.js.
• Replaced hard fallback to azure-openai with configured provider resolution:
  • config.modelProvider?.type || config.modelProvider?.provider || null
• Applies to Claude-family and GPT-family subagent models.

Testing

• Existing tests pass
• Added new tests
• Manual testing performed

Manual checks:

• npm run test:quick passed.
• Verified bug path/repro condition and fix behavior for non-Azure provider setups.

Checklist

• Code follows style guidelines
• Documentation updated
• Tests added/updated
• No new warnings

Notes

• Scope intentionally minimal (single-file bug fix).
• No secrets or local environment values are included.

[Plaidmustache]

Maintainer note: this is intentionally scoped to one bug-fix file (src/agents/executor.js) to keep review and rollback simple. Repro was non-Azure Task/subagent execution failing due to unintended Azure fallback. Validation run: npm run test:quick (pass).

[veerareddyvishal144]

Thanks @Plaidmustache for the contribution. This really means a lot