fix(#3717): context-length indicator honors provider per-model overrides (#3726) (#3752)

@rodboev. providers.<name>.models.<model>.context_length overrides (standard provider,
no base_url) were invisible to the session context resolver → wrong window shown/persisted,
could trip auto-compression at the wrong threshold. New _context_length_lookup_inputs_for_model
helper resolves provider config / base_url / custom_providers across route-load, session-save,
and SSE-usage paths; provider-scoped overrides match by provider name and forward as
config_context_length (returned before any base-url-gated probe).

Maintainer pre-merge items both already satisfied in PR head: no-base_url regression test
(test_route_resolver_uses_provider_model_context_length_without_base_url) present; session-save
_cfg_base_url assigned before the helper call (safe-bound, no NameError on TypeError fallback).
Verified api code byte-identical to PR head; 14 context-length tests pass. + CHANGELOG v0.51.300.

Co-authored-by: nesquena-hermes <[email protected]>

CHANGELOG.md

@@ -3,6 +3,11 @@
 
 ## [Unreleased]
 
+## [v0.51.300] — 2026-06-06 — Release JP (stage-3726 — context-length indicator honors provider per-model overrides)
+
+### Fixed
+- **The context-length indicator now honors per-model `context_length` overrides configured under a specific provider.** A `providers.<name>.models.<model>.context_length` override (standard provider, no `base_url`) was invisible to the session model's context resolver, so the usage indicator showed and persisted the wrong window — and could trip auto-compression at the wrong threshold. The resolver now collects the effective provider config, base URL, and custom-provider metadata across all three paths (route load, session save, live SSE usage), resolving provider-scoped overrides by provider-name match and forwarding them as `config_context_length` (which the agent returns before any base-URL-gated probe). `custom_providers`-with-`base_url` per-model overrides continue to resolve as before. (#3726 fixes #3717, @rodboev)
+
 ## [v0.51.299] — 2026-06-06 — Release JO (stage-3713 — update flow waits for a genuinely new server instance)
 
 ### Fixed