fix: surface provider-scoped context length in gateway session info

[guantik-gm]

Summary

• make GatewayRunner._format_session_info() reuse the provider compatibility layer when top-level model.context_length is absent
• surface per-model context_length from providers: / custom_providers: in the gateway session-info banner
• add regression coverage for both config shapes in tests/gateway/test_session_info.py

Problem

The gateway /reset / session-info banner only read top-level model.context_length, so configs that stored context under provider-scoped per-model entries (for example providers.<name>.models.<model>.context_length) could incorrectly fall back to the default 128K display even though runtime paths already honored the configured value.

Test Plan

• python -m pytest tests/gateway/test_session_info.py -q
• result: 11 passed

Notes

• preserves existing behavior for top-level model.context_length
• keeps fallback/default messaging unchanged when no config override is available

[alt-glitch]

Related to #10690, #5096, #8240, #14382 — all fix the same gateway session-info banner not reading provider-scoped context_length. Consider consolidating.

[nwang783]

I checked the related PRs called out above (#10690, #5096, #8240, #14382). They do appear to be solving the same user-visible bug family: the gateway session-info banner not surfacing the provider-scoped context_length correctly.

I don't see a reviewer-blocking code issue in this PR from that comment alone, but I do think we should consolidate before merge so we don't land the same fix multiple times with slightly different implementations/tests.

[teknium1]

Thanks for collecting fixes across the affected paths. The gateway-session-info portion is already on current main: gateway/run.py:12236-12237 supplies the compatibility-normalized provider entries to get_model_context_length(), whose per-model override path is at agent/model_metadata.py:2083-2096 (landed by 3ccf723bf). The related comments correctly identified this as a consolidation problem.

Problems

• The provider transport handling and background-review runtime inheritance are also already upstream (362996e26, e3901d5b2).
• The remaining MCP change uses the old mcp_<server>_<tool> construction. Main now centralizes MCP names in tools/mcp_tool.py:4477-4484 as mcp__<server>__<tool>, so this hunk needs adaptation rather than a direct cherry-pick.

Suggested changes

• Salvage the independent deterministic MCP length cap at mcp_prefixed_tool_name() and retain the current double-underscore convention; omit the already-landed gateway/runtime/review portions.

This is an automated hermes-sweeper review.

[alt-glitch]

This was generated by AI during triage.

Related to #18025 and the older session-info display work, but this live branch also modifies provider transport resolution, background-review inheritance, and MCP tool naming. It is not a duplicate; a split/consolidation decision would make the independent changes reviewable.

[alt-glitch]

This was generated by AI during triage.

Related to open display-only competitor #18025. This branch additionally changes custom-provider transport resolution, background-review credential inheritance, and MCP registry-name handling, so it is mixed-scope competing work rather than a duplicate; please split or consolidate.