fix(wtsite): copy the jsonld context before rewriting it

[LukasGold]

Fixes #177.

Changes

• WtSite._replace_jsonld_context_mapping deep-copies a dict argument at
  entry and rewrites the copy. The caller's object is no longer modified.
• The dict branch is no longer nested in an if. An argument of an
  unhandled type returns unchanged instead of falling through to an implicit
  None.
• New unit tests in tests/test_wtsite_jsonld_context_mapping.py.

Rationale

• The method returned a different kind of result per input type: the same
  string, a new list, the same mutated dict, or None. The new list was a
  shallow copy of the caller's dictionaries, which were rewritten in place, so
  it gave no protection.
• deepcopy rather than dict(context): the replaced values are nested
  dictionaries such as {"@id": ..., "@type": "@id"} and scoped @context
  blocks. A shallow copy repeats the defect one level down.
• The only caller in this repository, get_jsonld_context_loader, was safe by
  accident. WtPage.get_slot_content returns a fresh deepcopy on every call,
  so the loader rewrote a throwaway object. Nothing documented or tested that
  dependency.
• Contexts are small (66 to 77 keys for the merged Task chain on a live
  instance), so the copy is not a performance concern.

Verification

• 9 new tests pass.
• 7 of the 9 fail against the previous src/osw/wtsite.py, confirming they
  guard the reported behaviour. The 2 that pass assert the rewriting rules,
  which are unchanged.

[github-actions]

Release preview

Merging this PR would release v2.3.4 (current: v2.3.3).

Changelog preview (truncated)

## v2.3.4 (2026-09-18)

### Bug Fixes

- **wtsite**: Copy the jsonld context before rewriting it
  ([`ea0cddf`](https://github.com/OpenSemanticLab/osw-python/commit/ea0cddfa85a0025560dda49f78ff99577a86f526))

### Chores

- Relicense to Apache-2.0 ([#173](https://github.com/OpenSemanticLab/osw-python/pull/173),
  [`848615c`](https://github.com/OpenSemanticLab/osw-python/commit/848615cbd1e06401922eda88231a51028440a242))

_{Preview via python-semantic-release and conventional commits.}