fix(lmstudio): use detected context for runtime preload when no override is set

[lunarnexus]

Summary

• use the detected LM Studio context window for runtime preload when model.context_length is not explicitly set
• keep explicit model.context_length overrides authoritative
• add regression tests covering runtime preload target selection

Problem

Hermes resolved the larger LM Studio context window correctly for budgeting/display, but _ensure_lmstudio_runtime_loaded() still preloaded LM Studio models with:

target_ctx = max(config_context_length or 0, MINIMUM_CONTEXT_LENGTH)

When no explicit override was set, that forced the runtime load target to Hermes' 64K minimum even when get_model_context_length(...) resolved a larger LM Studio max_context_length.

In practice this meant:

• autodetection could report the correct larger context
• but LM Studio was still actually loaded at 64K

Fix

If model.context_length is explicitly set, keep using it.

Otherwise, resolve the model context via get_model_context_length(...) and use that detected value as the LM Studio preload target, floored at MINIMUM_CONTEXT_LENGTH only as a fallback.

Why this is separate from #45037

PR #45037 fixes the autodetection/reporting path when LM Studio is already loaded at the default 64K.

This PR fixes the runtime preload/load-target path that was still forcing LM Studio to load at 64K when no explicit override was set.

Tests

python -m pytest tests/agent/test_lmstudio_runtime_load.py -q -o addopts=''
python -m pytest tests/agent/test_model_metadata_local_ctx.py -q -o addopts=''

Related

• fixes [Bug]: lmstudio provider pre-loads models manually, bypassing JIT and trampling user context config #25989
• related to fix: use LM Studio max_context_length when model is loaded at default 64K #45037
• related to [Bug]: LM Studio custom_providers per-model context_length broken in 0.14.0 — regressed to 64K #30178

[lunarnexus]

Closing this in favor of the consolidated PR #52188, which includes this runtime preload fix plus the related LM Studio autodiscovery/reporting fix from #45037 in one reviewable patch.