docs: fix counter tutorial ownership semantics and add TXE tests

[critesjosh]

Summary

• The counter contract tutorial framed the example as "your own private counter," but increment(owner: AztecAddress) let any caller mutate any owner's counter. Drop the owner parameter from increment and read self.msg_sender() instead so a caller can only increment the counter mapped to their own address. Tutorial prose updated to match.
• Add a sibling counter_contract_test package (mirroring the existing logging_example_test layout) with TXE tests covering initialize and increment.

Why

The previous tutorial code taught a bad default — anyone can mutate anyone's counter. For the first contract tutorial that beginners copy-paste, the auth model matters more than any other change we could make to the example. This is a minimal, targeted fix; deliberately not a wholesale swap to the aztec-fundamentals-course version, which expands the surface to 7 functions (private + public + decrement + view) and is more appropriate as material for a follow-on tutorial.

What's not in scope

• No swap to PrivateMutable<UintNote> / public counters / decrement / #[view]. Those belong in a follow-on tutorial about private-vs-public state.
• The new counter_contract_test package is type = "lib", matching logging_example_test. docs/examples/bootstrap.sh compile only iterates type = "contract" packages, so the test package is in the workspace for syntax checking and reference but isn't yet invoked via aztec test in CI. Wiring tests into docs CI for both logging_example_test and counter_contract_test is a worthwhile follow-up.

Test plan

• CI's docs/examples/bootstrap.sh compile builds counter_contract against the new increment() signature
• aztec test against counter_contract_test passes locally (cannot verify here — the in-repo nargo binary is stale and the docs workspace doesn't compile against current aztec-nr)
• Docusaurus dev build renders the updated tutorial without #include_code errors