fix: Improve error messages for non-string templates in ConditionalRouter

[vblagoje]

Why:

Replaces cryptic "Can't compile non template nodes" errors with actionable guidance when users provide non-string values (e.g., 2 instead of "2") as route outputs in ConditionalRouter.

• fixes Better error reporting for invalid / non-template jinja2 template on ConditionalRouter #10127

What:

• Added type checking in _validate_template() to detect non-string templates before Jinja parsing
• Enhanced error messages in _validate_routes() to show value type and suggest fixes (e.g., "use '2' instead of 2")
• Added test cases verifying improved error messages for single and list outputs

How can it be used:

from haystack.components.routers import ConditionalRouter

# Before: Cryptic error
routes = [{"condition": "{{ True }}", "output": 2, "output_name": "num", "output_type": int}]
# ValueError: Can't compile non template nodes

# After: Clear guidance
routes = [{"condition": "{{ True }}", "output": 2, "output_name": "num", "output_type": int}]
# ValueError: Invalid template for output: 2 (type: int). Output must be a string representing a valid Jinja2 template. For example, use '2' instead of 2.

How did you test it:

• Added unit tests for non-string output templates (single and list cases)
• Verified error messages contain required guidance keywords
• Ran full ConditionalRouter test suite (28 tests passing)

Notes for the reviewer:

Error message improvements are backward compatible and only enhance user experience during initialization validation.

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1 Skipped Deployment

Project	Deployment	Preview	Comments	Updated (UTC)
haystack-docs	Ignored	Preview		Dec 10, 2025 9:23am

[vblagoje]

@mpangrazzi nothing urgent 🙏

[coveralls]

Pull Request Test Coverage Report for Build 19899324781

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 14 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.003%) to 92.196%

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Files with Coverage Reduction	New Missed Lines	%
components/routers/conditional_router.py	14	87.97%

Totals
Change from base Build 19898946186:	-0.003%
Covered Lines:	14082
Relevant Lines:	15274

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💛 - Coveralls

[sjrl]

I'm a little confused on this statement. When would 2! appear in the error message?

[sjrl]

One minor comment, otherwise looks good!