fix: ignore dimension for custom mock embeddings

[carey-bk]

Related Issues

Fixes #12978.

Proposed Changes

MockTextEmbedder and MockDocumentEmbedder rejected non-positive dimension
values even when a fixed embedding or embedding_fn made the parameter irrelevant.
Validate positive dimensions only in the default deterministic mode, matching the
documented contract. Preserve mutual exclusion and vector validation, clarify the
raises documentation, and add a release note.

The regression tests cover both components, zero and negative dimensions, actual
callback invocation, invalid vectors, and serialization round trips with importable
named functions. Supplied dimension values and the serialization format are unchanged.

How did you test it?

Latest upstream base checked: 8a5406eea71a0fc19e94c4b9a5cd96df2158a45a.

• Re-ran the focused mock embedder suite after rebasing: 79 passed.
• Re-ran hatch run fmt-check: lint and formatting passed.
• All applicable pre-commit hooks passed when creating the commit.
• The same regression tests previously produced 32 failed, 47 passed on the
  unmodified base and 79 passed with the fix.
• Additional local validation of this unchanged patch: the embedder unit suite
  passed with 148 passed, 6 integration tests deselected, and mypy passed on
  the four changed Python files.

Focused test command (Python 3.12, Hatch environment with the required dependencies):

hatch run python -m pytest --cov=haystack -m 'not integration' \
  --disable-socket --allow-unix-socket \
  test/components/embedders/test_mock_text_embedder.py \
  test/components/embedders/test_mock_document_embedder.py

Tests use fixed inputs and offline mocks, with network sockets disabled. No model
weights were downloaded or model APIs called.

Notes for the reviewer

This patch was fully generated with Codex. The results above are local; remote CI
has not run. In the local environment, whole-tree mypy reported the same four
unused-ignore diagnostics on the base and patched code. The full dependency/CI
matrix remains unverified. Full-history reno lint was blocked by the shallow
clone; the new note passed YAML, reStructuredText, and project hook checks.

Checklist

• Read the contributors guidelines and code of conduct.
• Updated the related issue with new insights and changes.
• Added regression coverage and updated the docstrings.
• Used a conventional commit type for the PR title.
• Documented the behavior and included a release note.
• Ran the applicable pre-commit hooks and addressed their findings.

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Coverage report

Click to see where and how coverage changed

File	Statements	Missing	Coverage	Coverage (new stmts)	Lines missing
haystack/components/embedders
mock_document_embedder.py
mock_text_embedder.py
Project Total

This report was generated by python-coverage-comment-action

[anakin87]

Thank you!

I simplified the tests a bit