fix(context): honor custom_providers context_length on /model switch + bump probe tier to 256K (#15844)

Fixes #15779. Custom-provider per-model context_length (`custom_providers[].models.<id>.context_length`) is now honored across every resolution path, not just agent startup. Also adds 256K as the top probe tier and default fallback.

## What changed

New helper `hermes_cli.config.get_custom_provider_context_length()` — single source of truth for the per-model override lookup, with trailing-slash-insensitive base-url matching.

`agent.model_metadata.get_model_context_length()` gains an optional `custom_providers=` kwarg (step 0b — runs after explicit `config_context_length` but before every other probe).

Wired through five call sites that previously either duplicated the lookup or ignored it entirely:
- `run_agent.py` startup — refactored to use the new helper (dedups legacy inline loop, keeps invalid-value warning)
- `AIAgent.switch_model()` — re-reads custom_providers from live config on every /model switch
- `hermes_cli.model_switch.resolve_display_context_length()` — new `custom_providers=` kwarg
- `gateway/run.py` /model confirmation (picker callback + text path)
- `gateway/run.py` `_format_session_info` (/info)

## Context probe tiers

`CONTEXT_PROBE_TIERS = [256_000, 128_000, 64_000, 32_000, 16_000, 8_000]` — was `[128_000, ...]`. `DEFAULT_FALLBACK_CONTEXT` follows tier[0], so unknown models now default to 256K. The stale `128000` literal in the OpenRouter metadata-miss path is replaced with `DEFAULT_FALLBACK_CONTEXT` for consistency.

## Repro (from #15779)

```yaml
custom_providers:
  - name: my-custom-endpoint
    base_url: https://example.invalid/v1
    model: gpt-5.5
    models:
      gpt-5.5:
        context_length: 1050000
```

`/model gpt-5.5 --provider custom:my-custom-endpoint` → previously "Context: 128,000", now "Context: 1,050,000".

## Tests

- `tests/hermes_cli/test_custom_provider_context_length.py` — new file, 19 tests covering the helper, step-0b integration, and the 256K tier invariants
- `tests/hermes_cli/test_model_switch_context_display.py` — added regression tests for #15779 through the display resolver
- `tests/gateway/test_session_info.py` — updated default-fallback assertion (128K → 256K)
- `tests/agent/test_model_metadata.py` — updated tier assertions for the new top tier

tests/agent/test_model_metadata.py

@@ -459,9 +459,10 @@ class TestGetModelContextLength:
 
     @patch("agent.model_metadata.fetch_model_metadata")
     def test_api_missing_context_length_key(self, mock_fetch):
-        """Model in API but without context_length → defaults to 128000."""
+        """Model in API but without context_length → defaults to the top
+        probe tier (currently 256K)."""
         mock_fetch.return_value = {"test/model": {"name": "Test"}}
-        assert get_model_context_length("test/model") == 128000
+        assert get_model_context_length("test/model") == CONTEXT_PROBE_TIERS[0]
 
     @patch("agent.model_metadata.fetch_model_metadata")
     def test_cache_takes_priority_over_api(self, mock_fetch, tmp_path):
@@ -814,14 +815,17 @@ 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_128k(self):
-        assert CONTEXT_PROBE_TIERS[0] == 128_000
+    def test_first_tier_is_256k(self):
+        assert CONTEXT_PROBE_TIERS[0] == 256_000
 
     def test_last_tier_is_8k(self):
         assert CONTEXT_PROBE_TIERS[-1] == 8_000
 
 
 class TestGetNextProbeTier:
+    def test_from_256k(self):
+        assert get_next_probe_tier(256_000) == 128_000
+
     def test_from_128k(self):
         assert get_next_probe_tier(128_000) == 64_000
 
@@ -841,8 +845,8 @@ class TestGetNextProbeTier:
         assert get_next_probe_tier(100_000) == 64_000
 
     def test_above_max_tier(self):
-        """Value above 128K should return 128K."""
-        assert get_next_probe_tier(500_000) == 128_000
+        """Value above 256K should return 256K."""
+        assert get_next_probe_tier(500_000) == 256_000
 
     def test_zero_returns_none(self):
         assert get_next_probe_tier(0) is None