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

gateway/run.py

@@ -4891,6 +4891,7 @@ class GatewayRunner:
         provider = None
         base_url = None
         api_key = None
+        custom_provs = None
 
         try:
             cfg_path = _hermes_home / "config.yaml"
@@ -4908,6 +4909,11 @@ class GatewayRunner:
                             pass
                     provider = model_cfg.get("provider") or None
                     base_url = model_cfg.get("base_url") or None
+                try:
+                    from hermes_cli.config import get_compatible_custom_providers
+                    custom_provs = get_compatible_custom_providers(data)
+                except Exception:
+                    custom_provs = data.get("custom_providers")
         except Exception:
             pass
 
@@ -4926,6 +4932,7 @@ class GatewayRunner:
             api_key=api_key or "",
             config_context_length=config_context_length,
             provider=provider or "",
+            custom_providers=custom_provs,
         )
 
         # Format context source hint
@@ -5601,6 +5608,7 @@ class GatewayRunner:
                             base_url=result.base_url or current_base_url or "",
                             api_key=result.api_key or current_api_key or "",
                             model_info=mi,
+                            custom_providers=custom_provs,
                         )
                         if ctx:
                             lines.append(f"Context: {ctx:,} tokens")
@@ -5748,6 +5756,7 @@ class GatewayRunner:
             base_url=result.base_url or current_base_url or "",
             api_key=result.api_key or current_api_key or "",
             model_info=mi,
+            custom_providers=custom_provs,
         )
         if ctx:
             lines.append(f"Context: {ctx:,} tokens")