feat: overhaul context length detection with models.dev and provider-aware resolution (#2158)

Replace the fragile hardcoded context length system with a multi-source
resolution chain that correctly identifies context windows per provider.

Key changes:

- New agent/models_dev.py: Fetches and caches the models.dev registry
  (3800+ models across 100+ providers with per-provider context windows).
  In-memory cache (1hr TTL) + disk cache for cold starts.

- Rewritten get_model_context_length() resolution chain:
  0. Config override (model.context_length)
  1. Custom providers per-model context_length
  2. Persistent disk cache
  3. Endpoint /models (local servers)
  4. Anthropic /v1/models API (max_input_tokens, API-key only)
  5. OpenRouter live API (existing, unchanged)
  6. Nous suffix-match via OpenRouter (dot/dash normalization)
  7. models.dev registry lookup (provider-aware)
  8. Thin hardcoded defaults (broad family patterns)
  9. 128K fallback (was 2M)

- Provider-aware context: same model now correctly resolves to different
  context windows per provider (e.g. claude-opus-4.6: 1M on Anthropic,
  128K on GitHub Copilot). Provider name flows through ContextCompressor.

- DEFAULT_CONTEXT_LENGTHS shrunk from 80+ entries to ~16 broad patterns.
  models.dev replaces the per-model hardcoding.

- CONTEXT_PROBE_TIERS changed from [2M, 1M, 512K, 200K, 128K, 64K, 32K]
  to [128K, 64K, 32K, 16K, 8K]. Unknown models no longer start at 2M.

- hermes model: prompts for context_length when configuring custom
  endpoints. Supports shorthand (32k, 128K). Saved to custom_providers
  per-model config.

- custom_providers schema extended with optional models dict for
  per-model context_length (backward compatible).

- Nous Portal: suffix-matches bare IDs (claude-opus-4-6) against
  OpenRouter's prefixed IDs (anthropic/claude-opus-4.6) with dot/dash
  normalization. Handles all 15 current Nous models.

- Anthropic direct: queries /v1/models for max_input_tokens. Only works
  with regular API keys (sk-ant-api*), not OAuth tokens. Falls through
  to models.dev for OAuth users.

Tests: 5574 passed (18 new tests for models_dev + updated probe tiers)
Docs: Updated configuration.md context length section, AGENTS.md

Co-authored-by: Test <test@test.com>

tests/agent/test_model_metadata.py

@@ -472,35 +472,35 @@ class TestContextProbeTiers:
         for i in range(len(CONTEXT_PROBE_TIERS) - 1):
             assert CONTEXT_PROBE_TIERS[i] > CONTEXT_PROBE_TIERS[i + 1]
 
-    def test_first_tier_is_2m(self):
-        assert CONTEXT_PROBE_TIERS[0] == 2_000_000
+    def test_first_tier_is_128k(self):
+        assert CONTEXT_PROBE_TIERS[0] == 128_000
 
-    def test_last_tier_is_32k(self):
-        assert CONTEXT_PROBE_TIERS[-1] == 32_000
+    def test_last_tier_is_8k(self):
+        assert CONTEXT_PROBE_TIERS[-1] == 8_000
 
 
 class TestGetNextProbeTier:
-    def test_from_2m(self):
-        assert get_next_probe_tier(2_000_000) == 1_000_000
-
-    def test_from_1m(self):
-        assert get_next_probe_tier(1_000_000) == 512_000
-
     def test_from_128k(self):
         assert get_next_probe_tier(128_000) == 64_000
 
-    def test_from_32k_returns_none(self):
-        assert get_next_probe_tier(32_000) is None
+    def test_from_64k(self):
+        assert get_next_probe_tier(64_000) == 32_000
+
+    def test_from_32k(self):
+        assert get_next_probe_tier(32_000) == 16_000
+
+    def test_from_8k_returns_none(self):
+        assert get_next_probe_tier(8_000) is None
 
     def test_from_below_min_returns_none(self):
-        assert get_next_probe_tier(16_000) is None
+        assert get_next_probe_tier(4_000) is None
 
     def test_from_arbitrary_value(self):
-        assert get_next_probe_tier(300_000) == 200_000
+        assert get_next_probe_tier(100_000) == 64_000
 
     def test_above_max_tier(self):
-        """Value above 2M should return 2M."""
-        assert get_next_probe_tier(5_000_000) == 2_000_000
+        """Value above 128K should return 128K."""
+        assert get_next_probe_tier(500_000) == 128_000
 
     def test_zero_returns_none(self):
         assert get_next_probe_tier(0) is None