fix: apply additional_context to Documents in the document path of extract()

[SuperMarioYL]

Description

When calling extract() with an iterable of Document objects, the top-level
additional_context parameter was silently ignored.

Root cause: extract() has two code paths:

• String path (working): wraps the string in Document(additional_context=additional_context), then calls annotate_text().
• Document path (broken): called annotate_documents() directly, never forwarding additional_context.

Fix

When a global additional_context is provided alongside a document iterable,
new Document instances are created for any document that has no per-document
context set. Per-document context always takes precedence over the global
value, and original caller objects are not mutated.

Key implementation notes:

• The iterable is only materialized when additional_context is not None;
  when it is None, the raw iterable is forwarded unchanged to preserve
  streaming behaviour for large inputs.
• Copied Documents preserve _document_id and _tokenized_text from the
  originals to avoid regenerating IDs or discarding cached tokenization.

Fixes #445

How Has This Been Tested?

Added 7 new unit tests to tests/extract_precedence_test.py covering:

• Global context applied to documents with no per-document context
• Per-document context takes precedence over the global value
• None additional_context leaves documents and iterable unchanged
• Explicit document IDs are preserved on copied documents
• Generator/lazy-iterable inputs work correctly
• Original caller Documents are not mutated
• Empty-string additional_context is treated as a non-None value

All 525 existing tests pass (excluding pre-existing env-dependent plugin test).

Checklist

• My code follows the style guidelines of this project
• I have self-reviewed my own code
• I have made corresponding changes to the documentation (n/a)
• My changes generate no new warnings
• I have added tests that prove my fix is effective
• New and existing unit tests passed locally with my changes (pytest tests/)
• I have run ./autoformat.sh on changed files

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[github-actions]

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[aksg87]

Hi @SuperMarioYL, please complete the CLA at https://cla.developers.google.com/ so we can review this PR. Without cla/google green we can't proceed. If we don't see it complete in the next couple of days, we'll close this out and handle the issue separately. Thanks!