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

hermes_cli/model_switch.py

@@ -533,6 +533,7 @@ def resolve_display_context_length(
     base_url: str = "",
     api_key: str = "",
     model_info: Optional[ModelInfo] = None,
+    custom_providers: list | None = None,
 ) -> Optional[int]:
     """Resolve the context length to show in /model output.
 
@@ -543,6 +544,11 @@ def resolve_display_context_length(
     about Codex OAuth, Copilot, Nous, and falls back to models.dev for the
     rest.
 
+    When ``custom_providers`` is provided, per-model ``context_length``
+    overrides from ``custom_providers[].models.<id>.context_length`` are
+    honored — this closes #15779 where ``/model`` switch ignored user-set
+    overrides.
+
     Prefer the provider-aware value; fall back to ``model_info.context_window``
     only if the resolver returns nothing.
     """
@@ -553,6 +559,7 @@ def resolve_display_context_length(
             base_url=base_url or "",
             api_key=api_key or "",
             provider=provider or None,
+            custom_providers=custom_providers,
         )
         if ctx:
             return int(ctx)