RagTokenizer: add encode and patch_token(_id) forwarding

[sherlock-488]

Fixes #35532

What does this PR do?

• Adds encode() to RagTokenizer by forwarding to current_tokenizer.
• Adds patch_token / patch_token_id getters and setters, also forwarding to current_tokenizer.
• Adds a lightweight regression test using dummy tokenizers (no model downloads).

Why?

RagTokenizer forwards __call__() via current_tokenizer but lacks common tokenizer APIs like encode() and patch_token(_id),
which can break preprocessing code expecting a standard tokenizer interface.

Tests

python -m pytest -q tests/models/rag/test_tokenization_rag_interface.py
make style

[github-actions]

[For maintainers] Suggested jobs to run (before merge)

run-slow: rag

[sherlock-488]

Hi! This PR implements encode() and patch_token/patch_token_id forwarding for RagTokenizer, with a lightweight regression test (no model downloads).
Local test: python -m pytest -q tests/models/rag/test_tokenization_rag_interface.py
It looks like 2 GitHub Actions workflows are awaiting maintainer approval — could a maintainer please approve them so CI can proceed?
Also, the bot suggested run-slow: rag before merge; happy to wait for that run if needed. Thanks!