fix(llm): delegate context_window() in SubProvider (#1473)

[bug-ops]

Summary

SubProvider in crates/zeph-llm/src/orchestrator/router.rs implements LlmProvider but did not override context_window(). The default trait implementation returns None, which caused a silent chain failure: auto_budget_tokens() returned 0, context_manager.budget was set to None, and prepare_context() skipped all context enrichment — semantic recall, graph recall, summaries, and cross-session memories never ran when using the orchestrator provider.

Changes

• Added context_window() delegation to impl LlmProvider for SubProvider
• Added 4 unit tests covering Ollama, Claude, and OpenAi variants

Related

• Fixes SubProvider::context_window() returns None — breaks auto_budget and graph recall #1473
• Follow-up for remaining missing delegations: fix(llm): delegate missing LlmProvider methods in SubProvider #1497

Test plan

• cargo +nightly fmt --check passes
• cargo clippy --workspace --features full -- -D warnings passes
• cargo nextest run --config-file .github/nextest.toml --workspace --features full --lib --bins passes (4941/4941)
• All 4 new sub_provider_context_window_* tests pass