fix(hindsight): surface the exception chain when the local runtime is unavailable

[falkoro]

What does this PR do?

Diagnosability fix for the failure mode documented in #60783's second confirming report: when the local Hindsight runtime fails to import, _check_local_runtime() returned str(exc) — which drops the exception cause/context chain. Upstream, hindsight_api's LocalSTEmbeddings.initialize() catches the real error and re-raises a generic "sentence-transformers is required … pip install sentence-transformers" even when that package is installed — during the #60783 huggingface-hub downgrade the actual conflict (huggingface-hub>=1.5.0,<2.0 is required … found 1.2.3) lived only in the chained cause, and the surfaced message sent users reinstalling the wrong package.

Fix: a _format_exception_chain() helper walks __cause__/__context__ (cycle-safe, capped at 5 links) and renders the chain into the reason — which flows verbatim into _get_client()'s RuntimeError and availability messaging — plus a debug-level full traceback. The root cause becomes visible in one Hermes log read instead of requiring a dig through the Hindsight daemon's own profile logs.

Kept deliberately separate from #60797 (which fixes the downgrade itself) so each PR stays single-concern.

Related Issue

Diagnosability follow-up to #60783 (root cause fixed by #60797)

Type of Change

• 🐛 Bug fix (non-breaking change that fixes an issue)

Changes Made

• plugins/memory/hindsight/__init__.py — new _format_exception_chain(); _check_local_runtime() returns the rendered chain and logs the traceback at debug level
• tests/plugins/memory/test_hindsight_provider.py — 5 new tests: explicit from e chain, implicit __context__ chain, single-exception shape, cycle safety, and the end-to-end _check_local_runtime reason containing the root cause

How to Test

1. pytest tests/plugins/memory/test_hindsight_provider.py -q → 121 passed (5 new)
2. Manual: with a broken hindsight import (e.g. the lazy_deps: huggingface-hub==1.2.3 pin conflicts with transformers >=1.5.0 requirement, breaks Hindsight local embeddings #60783 downgrade state), hermes memory init now reports ImportError: sentence-transformers is required … <- caused by: ImportError: huggingface-hub>=1.5.0,<2.0 is required … instead of only the misleading first half.

Checklist

Code

• I've read the Contributing Guide
• My commit messages follow Conventional Commits
• I searched for existing PRs to make sure this isn't a duplicate (offered on the lazy_deps: huggingface-hub==1.2.3 pin conflicts with transformers >=1.5.0 requirement, breaks Hindsight local embeddings #60783 thread first; no objections, no competing PR)
• My PR contains only changes related to this fix
• I've run the test suite listed above and all tests pass
• I've added tests for my changes
• I've tested on my platform: Linux (CachyOS, Python 3.12)

Documentation & Housekeeping

• Relevant documentation — rationale documented in the helper docstring; N/A otherwise
• cli-config.yaml.example — N/A (no config keys)
• Cross-platform impact — pure Python exception formatting; platform-independent

[teknium1]

Thanks for the narrowly scoped diagnosability fix. Current main still drops chained import failures at plugins/memory/hindsight/__init__.py:140, and that reason reaches both the local-mode warning (:1273-1278) and _get_client() RuntimeError (:1016-1021). The formatter and focused explicit-cause, implicit-context, single-error, cycle, and integration-path tests are appropriate.

Problems

• tests/plugins/memory/test_hindsight_provider.py:1882 imports importlib as _importlib, but the test only uses hs.importlib; _importlib is unused.

Suggested changes

• Remove the unused import at tests/plugins/memory/test_hindsight_provider.py:1882.

Automated hermes-sweeper review.

[teknium1]

_importlib is never referenced; the test patches hs.importlib below. Please remove this unused import.