Anthropic Messages API: add extended (1h) prompt cache TTL behind experiment (#316178)

Re-introduces `cache_control.ttl: "1h"` for the Anthropic Messages
API tools + system breakpoints, gated on the main agent conversation
where the 2x cache-write cost trades favourably against the longer
hit window. Previously reverted from the copilot-chat repo.

All four gates must hold:
- Model is a 1M-context Claude variant (`claude-opus-4-{6,7}-1m...`)
- Setting `github.copilot.chat.anthropic.promptCaching.extendedTtl` is
  on (ConfigType.ExperimentBased, default false, advanced/experimental/onExp)
- Location is `ChatLocation.Agent` (Panel/Editor/Terminal/Notebook/
  EditingSession/Other and both proxy locations are excluded)
- Request is not a subagent (typed via
  `interactionTypeOverride === 'conversation-subagent'`, the same
  source of truth as the `X-Interaction-Type` wire header)

When all gates pass:
- The `extended-cache-ttl-2025-04-11` beta header is added
- The last non-deferred tool and the last system block carry
  `cache_control: { type: 'ephemeral', ttl: '1h' }`. The two rolling
  message breakpoints keep the default 5m TTL, satisfying Anthropic's
  longer-TTLs-before-shorter ordering rule.

Tests: messagesApi.spec.ts now at 65 tests (was 59); adds dedicated
`modelSupportsExtendedCacheTtl` and `isExtendedCacheTtlEnabled`
suites covering every gate explicitly.