feat: Add variable to specify inputs as optional to ConditionalRouter

[vblagoje]

Why:

Enhances the ConditionalRouter component by adding support for optional parameters, enabling default/fallback routing behavior when certain inputs are not provided at runtime.

• fixes Add variable to specify inputs as optional to ConditionalRouter #8556

What:

Introduces optional_variables parameter to specify which route variables can be omitted
Improves documentation with clear examples of fallback routing patterns
Implements proper serialization/deserialization for the new field

How can it be used:

Define routes with fallback behavior when certain parameters are missing:

from haystack import Pipeline
from haystack.components.routers import ConditionalRouter

routes = [
    {
        "condition": '{{ path == "rag" }}',
        "output": "{{ question }}",
        "output_name": "rag_route",
        "output_type": str
    },
    {
        "condition": "{{ True }}",  # fallback route
        "output": "{{ question }}",
        "output_name": "default_route",
        "output_type": str
    }
]

router = ConditionalRouter(routes, optional_variables=["path"])
pipe = Pipeline()
pipe.add_component("router", router)

# When 'path' is provided in the pipeline:
result = pipe.run(data={"router": {"question": "What?", "path": "rag"}})
assert result["router"] == {"rag_route": "What?"}

# When 'path' is not provided, fallback route is taken:
result = pipe.run(data={"router": {"question": "What?"}})
assert result["router"] == {"default_route": "What?"}

How did you test it:

Unit tests verify both direct component usage and pipeline integration
Tests cover simple and complex scenarios with and without optional parameters

[coveralls]

Pull Request Test Coverage Report for Build 11974160625

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

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Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 18 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.04%) to 90.285%

---

Files with Coverage Reduction	New Missed Lines	%
components/routers/conditional_router.py	18	86.15%

Totals
Change from base Build 11972705853:	0.04%
Covered Lines:	7881
Relevant Lines:	8729

---

💛 - Coveralls

[vblagoje]

@silvanocerza @sjrl - if I understood correctly the originating issue this seems like a relatively simple fix. Added a bunch of tests, maybe a bit more than needed just to make sure as this is an important component.

[sjrl]

Good example! Except this example actually works pre your changes. To get the error we were originally seeing you would need to put it in a Pipeline first. Even a single component pipeline would do. Otherwise it's a bit misleading here since this example would actually work without specifying the optional_variables parameter.

[sjrl]

Could we use the logger.warning here instead or in addition to? Using the logger allows us to surface these warnings to users in dC more easily.

[sjrl]

Yeah this looks like it solves the issue thanks @vblagoje !