fix: fix serialization of DocumentRecallEvaluator

[anakin87]

Related Issues

• similar to (De-) Serialization is not properly working for HuggingFaceAPITextEmbedder #7660

this fails because the RecallMode Enum is not YAML-serializable:

from haystack import Pipeline
from haystack.components.evaluators.document_recall import DocumentRecallEvaluator

p = Pipeline()
p.add_component("eval", DocumentRecallEvaluator())
dump = p.dumps()

p2 = Pipeline.loads(dump)

Proposed Changes:

• add to_dict method to handle the serialization

How did you test it?

CI, new unit tests, manual test

Checklist

• I have read the contributors guidelines and the code of conduct
• I have updated the related issue with new insights and changes
• I added unit tests and updated the docstrings
• I've used one of the conventional commit types for my PR title: fix:, feat:, build:, chore:, ci:, docs:, style:, refactor:, perf:, test:.
• I documented my code
• I ran pre-commit hooks and fixed any issue

[coveralls]

Pull Request Test Coverage Report for Build 9002627521

Warning: This coverage report may be inaccurate.

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Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.03%) to 90.365%

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Totals
Change from base Build 8938725904:	0.03%
Covered Lines:	6518
Relevant Lines:	7213

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💛 - Coveralls

[vblagoje]

LGTM, left one minor optional comment