feat: add context bloat detection rail

[MuneezaAzmat]

Summary

• Adds a new context_bloat_detection guardrail that detects context-manipulation attacks (padded, oversized, or repetitive content in tool outputs, RAG chunks, or user input)
• Checks (cheapest first): size cap, Shannon entropy, longest char run, n-gram repetition
• Supports reject, truncate, and warn actions via config
• Registers ContextBloatDetectionConfig Pydantic model in RailsConfigData with sensible defaults

Test plan

• Verify config loads with default values
• Verify oversized, low-entropy, padded, and repetitive inputs are detected
• Verify truncate mode truncates to max_chars
• Verify warn mode logs but does not block
• Verify normal text passes all checks

[m-misiura]

Thanks for the PR. To me the concept of using this rail to detect suspiciously long texts appears sound, although I think there might be a considerable overlap between the n-gram and the entropy method (I could be missing something though). You might also consider if you want to land this upstream.

In any case, from an engineering side, we'll need:

• test files: there is none currently
• config is always always instantiated, which breaks the optional pattern: IIUC, this rail is configured even when the user didn't ask for it
• there is no __init__.py: meaning the library won't be importable
• truncation happens after full analysis

Happy to expand if anything is unclear

[MuneezaAzmat]

Thanks for the comments @m-misiura :

Entropy catches character-level padding , while n-gram catches phrase-level repetition
example of what entropy will catch but n-gram wont and vice versa:
───────────────
Text: "ababab..."
Entropy: 1.000 🚩
Repetition Ratio: 0.000
───────────────
Text: "The quick brown fox jumps over" x30
Entropy: 4.109
Repetition Ratio: 0.966 🚩
───────────────

1. pushed the test file i used to verify - not sure if more tests are needed
2. kept default_factory to match every other rail in RailsConfigData. Afaiu It doesn't activate the rail , rail only runs when the user explicitly wires it in the flow.