feat(adk): expose current messages to middleware

[March-77]

What type of PR is this?

feat

Check the PR title.

• This PR title matches the required format.
• The description is user-oriented and clear.
• No separate documentation PR is required; the new API has Go documentation.

(Optional) Translate the PR title into Chinese.

feat(adk): 向中间件开放当前消息列表

(Optional) More detailed description for this PR(en: English/zh: Chinese).

en:

ChatModelAgent middleware hooks that do not receive TypedChatModelAgentState currently have no first-class way to inspect the current message list. Consumers must reach into compose.ProcessState and the deprecated concrete adk.State; this is impossible for AgenticMessage because its internal state type is intentionally unexported.

This change adds:

• GetMessages(ctx) for the standard *schema.Message path
• TypedGetMessages[M](ctx) for both supported message types
• a shallow snapshot contract so changing the returned slice cannot mutate agent state
• clear errors when called outside an active ChatModelAgent execution

The skill middleware now consumes the public API instead of deprecated/internal state details, which also removes its message-state access restriction for the Agentic path.

User impact: middleware and tool hooks can inspect current messages without depending on compose internals. Existing APIs and behavior are unchanged.

Compatibility/risk: additive public API only. Message objects are intentionally shared and documented as read-only; only the returned slice is copied.

Validation:

• go test ./adk -run "TestGetMessages|TestAgenticChatModelAgentRun_WithMiddleware" -count=10
• go test -race ./adk -run "TestGetMessages|TestAgenticChatModelAgentRun_WithMiddleware" -count=1
• go test ./adk/middlewares/skill
• go test ./adk
• go test ./...
• go vet ./...

golangci-lint could not be completed locally because the pinned runner repeatedly stalled while downloading github.com/MirrexOne/unqueryvet v1.5.4.

(Optional) Which issue(s) this PR fixes:

Fixes #868

[March-77]

Revalidation update (2026-08-23): this remains mergeable against current main (ebd616c), with no equivalent GetMessages/TypedGetMessages API on main or alpha/10. The two main commits since this PR's base touch unrelated middleware packages.

Local validation passes:

• go test ./adk ./adk/middlewares/skill
• go test -race ./adk -run 'TestGetMessages|TestAgenticChatModelAgentRun_WithMiddleware' -count=1

The PR still matches the first-class API direction requested in #868 and removes the skill middleware's Agentic fork-mode restriction. GitHub Actions have never started for this fork PR (CLA is passing), so the blocked state is required review rather than a failing check. A maintainer review/workflow approval when convenient would be appreciated.