chore: three small clean-ups in one PR

[Lancetnik]

Description

Two clean-ups too small to review on their own, extracted from #3030 so the feature branch stops carrying them:

• PATH_CONTEXT_PREFIX — the context prefix a Path() parameter reads from ("message.path.") has exactly one definition, in faststream/_internal/constants.py, and Path() reads it from there. It was spelled inline before.
• parser_for(version) in faststream/mqtt/parser.py — the MQTT Subscriber and the in-memory MQTT test broker choose the parser class for a Broker version through this one function. Each picked it on its own before, and the two have to agree on which version they speak.

The third item the ticket lists, the .gitignore entries for local agent tooling and the graphify output, is already on main, so it is not part of this PR.

Nothing a user can see changes. No new tests: the change is one definition replacing two identical ones, and the existing MQTT and Path() suites pin the behaviour.

Part of the prefactor plan for #2451.

Type of change

• Refactor (no functional change)

Checklist

• My code adheres to the style guidelines of this project (just lint shows no errors)
• I have conducted a self-review of my own code
• My changes do not generate any new warnings
• Existing tests pass: tests/utils/context/test_path.py, tests/brokers/mqtt/test_path.py, tests/brokers/mqtt/test_testclient.py, tests/docs/mqtt/message/test_message.py (-m "not connected"; MQTT connected tests run in CI only)
• just mypy passes

🤖 Generated with Claude Code

[borisalekseev]

Better to narrow the mqtt version string type to Literal["3.1.1", "5.0"] and use typing.assert_never. This will simplify the work when adding a protocol version, and mypy will highlight places where corrections need to be made.

[Lancetnik]

Done in the second commit.

• MQTTVersion = Literal["3.1.1", "5.0"] lives once, in faststream/mqtt/parser.py, and MQTTBroker, MQTTRouter (FastAPI), MQTTBrokerConfig and the test broker spell their version through it.
• parser_for and the producer choice in MQTTBroker.__init__ switch over it with typing_extensions.assert_never (the project supports 3.10, so not typing.assert_never). Adding a version is now an edit to the alias, and mypy names both switches.
• The one place a third value exists is the config: version: MQTTVersion | Literal["unset"], "unset" being a Subscriber declared on a Router before a Broker composes its version in. The Subscriber resolves that to 5.0 explicitly, where the comment already explained it, instead of the previous getattr(..., "5.0") swallowing it.

just mypy and just pyright are clean; the MQTT test files pass locally without connected.