Fix concept-code lookups containing slashes

[konstjar]

Summary

• percent-encode vocabulary IDs and concept codes as individual URL path segments
• preserve slash-containing ICD-O codes such as 8032/3 as a single route parameter
• cover synchronous and asynchronous clients with regression tests

Validation

• uv run --extra dev pytest tests/unit/resources/test_concepts.py -q (40 passed)
• uv run --extra dev ruff check src/ tests/unit/resources/test_concepts.py
• uv run --extra dev ruff format --check src/ tests/unit/resources/test_concepts.py
• uv run --extra dev mypy src/omophub

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Summary by cubic

Fixes concept lookups so codes containing slashes, like ICD-O 8032/3, are sent as one URL path segment instead of being split into multiple parameters.

• Percent-encodes vocabulary IDs and concept codes in get_by_code for both sync and async clients.
• Clarifies in the docstrings that vocabulary_id and concept_code must be raw, URL-unencoded values.
• Adds regression tests covering slash-containing codes for both clients.

^{Written for commit 28a168a. Summary will update on new commits.}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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[cubic-dev-ai]

All reported issues were addressed across 2 files

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