feat(loop): configurable session-affinity header names

[zhanghanduo]

Why

bind_session_id hardcodes x-upstream-session-id, a header the old EAS gateway used for session affinity. llm-hub, the company proxy most of our LLM traffic goes through, pins a session to one upstream key in its account pool using its own header, X-Llmhub-Session. Without that header, each turn of a conversation can land on a different key and miss the prompt cache: the same work takes several times longer and costs several times more. AgentCore should not know about llm-hub, so the header name becomes a setting the host supplies.

What

• bind_session_id(..., header_names=DEFAULT_SESSION_HEADER_NAMES): every name in the list gets the session id. An empty tuple sends nothing. A bare str raises TypeError, because iterating it would send one header per character.
• AgentLoopHooks.session_header_names: sets the header names for the loop's built-in binding. Like sticky_session_enabled, it is ignored when bind_session is supplied.
• DEFAULT_SESSION_HEADER_NAMES is exported. The default is unchanged, so this is fully backward compatible.
• Ships in the unreleased 0.11.1 alongside fix(providers): forward default_headers to Anthropic/Bedrock clients #38 (not yet tagged or published). The version stays at 0.11.1, so this PR carries the skip-version-bump label. The change is additive: one new keyword with a default and one new hook field with a default.

Consumer follow-up

ApodexHarness passes ("x-upstream-session-id", "X-Llmhub-Session") from its _bind adapter and hook wiring.

Tests

• Unit test: multiple header names, an empty tuple, and rejection of a bare str.
• Loop test: a header name set on the hook reaches the provider call's extra_headers.
• Full suite: 1576 passed. ruff, pyright, and the unconsumed-fields and version-bump checks pass locally.

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