Test PIIMiddleware stream redaction at every chunk split

Submission checklist

• This is a feature request, not a bug report or usage question.
• I added a clear and descriptive title that summarizes the feature request.
• I used the GitHub search to find a similar feature request and didn't find it.
• I checked the LangChain documentation and API reference to see if this feature already exists.
• This is not related to the langchain-community package.

Package (Required)

• langchain
• langchain-openai
• langchain-anthropic
• langchain-classic
• langchain-core
• langchain-model-profiles
• langchain-tests
• langchain-text-splitters
• langchain-chroma
• langchain-deepseek
• langchain-exa
• langchain-fireworks
• langchain-groq
• langchain-huggingface
• langchain-mistralai
• langchain-nomic
• langchain-ollama
• langchain-openrouter
• langchain-perplexity
• langchain-qdrant
• langchain-typesafe
• langchain-xai
• Other / not sure / general

Feature Description

One unit test for PIIMiddleware's in-flight stream redaction (#37616) that checks every chunk split point, not a few chosen ones.

Use Case

#37616 redacts streamed text in flight. Its tests check a few chosen splits (test_pii_split_across_deltas_is_caught, test_pii_straddling_lookback_boundary_is_caught, test_credit_card_split_across_whitespace_is_caught). A later change to the lookback logic that breaks a different split could go unnoticed.

Proposed Solution

Add one test to test_pii.py that streams a short text through _PIIStreamTransformer split into two deltas at every position, and checks the output matches the unsplit stream. The text has an email, a card number and a value next to non-ASCII text. About 20 lines, test-only, no new dependencies. When it fails it names the split.

@mdrxy would you take this? If yes, please assign me and I'll open the PR.

Alternatives Considered

No response

Additional Context

Related: #35011, #37616

Social handles (optional)

No response