Mark tiktoken functions as IMMUTABLE by default

[nick-inkeep]

Description

When using tiktoken_count in a generated column on managed Postgres instances (like Neon), we encounter:

ERROR: generation expression is not immutable

Since tokenization is deterministic (same input → same output), marking these functions as IMMUTABLE in the extension itself would allow direct use in generated columns without requiring superuser privileges.

Example Use Case

CREATE TABLE documents (
    content text NOT NULL,
    token_count integer GENERATED ALWAYS AS (tiktoken_count('cl100k_base', content)) STORED
);

Benefits

• Works out of the box on managed Postgres instances
• Enables use in generated columns
• Follows Postgres best practices for deterministic functions