types: add the Any type-URL helpers to AnyView

[farmer-data]

Fixes #643.

AnyView gains inherent type_name, is_message, is_type, unpack_message and unpack_if, so a zero-copy handler can test and unpack the packed type without converting to an Any or splitting type_url by hand.

• Any::type_name and AnyView::type_name now share one private type_name_of, so the rules can't drift, including the bare-name reading from any: read a type URL without a '/' as a bare message name #692. AnyView::type_name returns Option<&'a str>, borrowing from the decoded buffer rather than the view.
• unpack_message and unpack_if decode an owned T from the borrowed value. For a view of the payload, the docs point to is_message::<FooView>() plus FooView::decode_view(view.value). A view-returning unpack_view would be additive, and I left it out since Any has no counterpart.
• Tests: a table test checks every helper against Any on 8 type URLs (empty, trailing /, no /, a near-miss name, other prefixes), plus lifetime, view-type and decode-error tests. The guide's Any section gets one paragraph, including how to reach the helpers from AnyOwnedView via view().

Verification: task lint, task test-local (4293 passed, 0 failed) and strict task doc pass with the repo-pinned protoc 33.5. Reviewed with rust-code-reviewer and rust-api-ergonomics-reviewer per CLAUDE.md: no Critical or High findings. The Medium (the is_type doc wording) and the doc Lows are addressed.

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