AI: persist only the pinned field, not the whole config block

Adversarial review of the previous commit found a real regression it
introduced, reproduced against the actual code rather than inferred.

Changing _persist_active_ai_config from setdefault("configs", ...) to a
direct assignment made every backend switch write the whole in-memory
AI_CONFIGS over the settings file. Because AI_CONFIGS is now the built-in
defaults merged UNDER the file, that meant:

* an operator's hand edits were destroyed - and hand editing is the only
  way to change cheap_model / temperature / max_tokens, since
  set_active_model writes latest_model and there is no command for the rest,
* a config deliberately deleted from the file was re-seeded from the
  defaults and written back, permanently,
* pinning a model for one provider silently reverted another provider's
  entry,
* CONJURER_OLLAMA_MODEL stopped having any effect once the env-derived
  block had been persisted once.

The original motivation was still valid (plain setdefault would drop a
pinned model), so the fix is narrower rather than a revert: persist ONLY
the field this process actually changed. _persist_active_ai_config takes
model_for and writes back just that config's latest_model; everything else
in the on-disk block is left exactly as found. The constants.py merge stays
- it is what keeps a newly added provider visible after an upgrade - and is
now in-memory only, so it cannot reach the file.

Tests: the disk-write path had ZERO coverage, which is precisely how this
got in. Added four tests that drive the real _persist_active_ai_config
against a temp settings file: the pin lands while operator edits survive and
a deleted config is not resurrected; a plain switch leaves the configs block
byte-identical; a pin survives a re-read; a corrupt file does not raise.
Verified they have teeth - reintroducing the regression fails two of them.

Also hardened two weak tests the review caught: the pin test asserted on the
object set_active_model returns, which IS the mutated dict (so it passed
regardless), and the unconfigured-endpoint test monkeypatched OLLAMACLIENT
to None when it was already None, passing vacuously.

Suite: 72 unit + 70 integration green.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

ai_functions.py

@@ -137,11 +137,11 @@ def set_active_model(model: str, name: str = None) -> dict:
         raise ValueError("pusta nazwa modelu")
     cfg = AI_CONFIGS[cfg_name]
     cfg["latest_model"] = model.strip()
-    _persist_active_ai_config(_ACTIVE_CONFIG_NAME)
+    _persist_active_ai_config(_ACTIVE_CONFIG_NAME, model_for=cfg_name)
     return cfg
 
 
-def _persist_active_ai_config(name: str) -> None:
+def _persist_active_ai_config(name: str, model_for: str = None) -> None:
     """Best-effort write of the active-config choice into system_gpt_settings.json.
 
     Keeps the historical two-element structure intact: updates index 2 if it
@@ -161,10 +161,16 @@ def _persist_active_ai_config(name: str) -> None:
         return
     if len(data) > 2 and isinstance(data[2], dict):
         data[2]["active"] = name
-        # Assign (not setdefault): AI_CONFIGS is the in-memory truth and may
-        # carry a model pinned via set_active_model, which setdefault would
-        # silently drop on restart.
-        data[2]["configs"] = AI_CONFIGS
+        # Write back ONLY what this process actually changed. Assigning the whole
+        # in-memory AI_CONFIGS here would clobber operator hand-edits (the only
+        # way to change cheap_model/temperature/max_tokens) and re-seed configs
+        # deliberately deleted from the file, because AI_CONFIGS is the built-in
+        # defaults merged under the file. setdefault alone is not enough either:
+        # it would drop a model pinned via set_active_model, hence model_for.
+        block = data[2].setdefault("configs", AI_CONFIGS)
+        if model_for and model_for in AI_CONFIGS:
+            entry = block.setdefault(model_for, dict(AI_CONFIGS[model_for]))
+            entry["latest_model"] = AI_CONFIGS[model_for]["latest_model"]
     else:
         data = data[:2] + [{"active": name, "configs": AI_CONFIGS}]
     try: