fix(sdk): decrypt encrypted fallback profiles

[Sehlani042]

HUMAN:

---

AGENT:

Why

Encrypted LLM profiles are decrypted on the main conversation path, but fallback profiles are resolved lazily through a separate LLMProfileStore that did not receive the conversation cipher. As a result, FallbackStrategy could pass a Fernet ciphertext (gAAAAA...) to the provider as an API key. This also affected strategies that had cached a fallback before being attached to a conversation.

Summary

• Allow LLMProfileStore to retain an optional default cipher while preserving explicit per-load cipher precedence.
• Bind the conversation cipher to FallbackStrategy and invalidate previously resolved fallback/profile-store caches when the cipher changes.
• Add a deterministic regression covering encrypted fallback resolution before and after conversation binding.

Issue Number

Fixes #4558

How to Test

Reproduction and regression:

1. Save a fallback LLM profile with include_secrets=True and a Cipher.
2. Resolve it through an unbound FallbackStrategy; the old path exposes the encrypted gAAAAA... token.
3. Create LocalConversation(..., cipher=cipher) with that strategy.
4. Resolve the fallback again and verify the API key is the original plaintext value.

Commands and results:

uv run pytest -q \
  tests/sdk/llm/test_llm_fallback.py \
  tests/sdk/llm/test_llm_profile_store.py \
  tests/sdk/conversation/test_switch_model.py
# 110 passed

uv run pre-commit run --files \
  openhands-sdk/openhands/sdk/conversation/impl/local_conversation.py \
  openhands-sdk/openhands/sdk/llm/fallback_strategy.py \
  openhands-sdk/openhands/sdk/llm/llm_profile_store.py \
  tests/sdk/llm/test_llm_fallback.py
# Ruff format/lint, PEP8, Pyright, import rules, and Tool registration passed

uv run pytest -q tests/sdk
# 5948 passed, 9 skipped, 10 xfailed, 2 failed

The same two full-suite failures reproduce on untouched origin/main: LiteLLM now reports support for moonshot/kimi-k3, and the local nonexistent MCP endpoint returns HTTP 503. No additional failure is introduced by this patch.

git diff --check also passes.

Video/Screenshots

Not applicable; this is an SDK behavior fix covered by a deterministic executable regression.

Design Doc

Not included; the change is limited to cipher propagation and cache invalidation across existing SDK objects.

Type

• Bug fix
• Feature
• Refactor
• Breaking change
• Docs / chore

Notes

Codex assisted with issue triage, implementation, and test execution. The patch was constrained to the fallback-profile cipher path and checked for competing pull requests before publication.

[github-actions]

📁 PR Artifacts Notice

This PR contains a .pr/ directory with temporary PR-specific documents. Because this is a fork PR, the directory will be automatically removed from main immediately after merge.