fix: Add warm_up() calls to tests after ChatGenerator tool initialization changes

[vblagoje]

Why:

Fixes failing tests on main branch after PR #9942 which introduced the warm_up() method requirement for ChatGenerator components when using tools. Tests were failing because Agent and LLMMessagesRouter components now require explicit warm-up calls before execution to properly initialize tools.

What:

• Added agent.warm_up() calls in test_agent_breakpoints.py before agent execution with tools
• Added router.warm_up() call in test_llm_messages_router.py before router execution with tools
• Added agent.warm_up() calls (2 locations) in test_pipeline_tool.py for both sync and async test cases
• Created Reno release note documenting the breaking change and required user action

How can it be used:

After PR #9942, all Agent and LLMMessagesRouter usage with tools must call warm_up() before execution:

# Before (now fails):
agent = Agent(llm=OpenAIChatGenerator(model="gpt-4"), tools=[my_tool])
result = agent.run(messages=[ChatMessage.from_user("Hello")])

# After (required pattern):
agent = Agent(llm=OpenAIChatGenerator(model="gpt-4"), tools=[my_tool])
agent.warm_up()  # Required for tool initialization
result = agent.run(messages=[ChatMessage.from_user("Hello")])

# Works for async as well:
agent.warm_up()
result = await agent.run_async(messages=[ChatMessage.from_user("Hello")])

How did you test it:

• Updated all affected test files to include warm_up() calls
• Verified tests pass with the new pattern
• Created release note with code examples for user migration

Notes for the reviewer:

This is a direct follow-up to PR #9942 which introduced tool warm-up for ChatGenerators. The release note is categorized as a fix rather than upgrade since it documents the resolution of the test failures and provides migration guidance. All changes are minimal—just adding the required warm_up() calls before component execution.

Related Issues

• Follows PR #9942

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1 Skipped Deployment

Project	Deployment	Preview	Comments	Updated (UTC)
haystack-docs	Ignored	Preview		Oct 29, 2025 11:29am

[coveralls]

Pull Request Test Coverage Report for Build 18906379731

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 92.235%

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Totals
Change from base Build 18904192695:	0.0%
Covered Lines:	13494
Relevant Lines:	14630

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💛 - Coveralls

[sjrl]

Looks good, just an update to the reno!