Resolve the MCP session name from a single source in the alarm path

[RhysSullivan]

Problem

The MCP session Durable Object's alarm could die reading the session id it had
just decided existed.

PartyServer resolves a DO's name from three places, and does not consult them in
one place: the name getter reads ctx.id.name and an in-memory field that is
hydrated only during initialization, while the durable __ps_name record is
read only by that initialization. The alarm handler makes most of its
decisions itself and delegates to super.alarm() on one branch only, so it
never runs that hydration.

The alarm's unaddressable-session guard read the durable record directly. On an
invocation without a ctx.id.name it therefore concluded the session was
addressable, continued — and threw on the very next read of the session id,
which was there to put an id in a log line about the decision it had just
made. The lease branches log before they act, so the extension was lost with the
line: the alarm died, retried, and the session stayed pinned without making
progress.

Fix

One accessor now answers "what is this DO's name", consulting ctx.id.name, the
in-memory field, and the durable record, with a defined fallback order. The
alarm resolves the name through it up front and remembers the answer, so the
guard and every read after it agree by construction.

Separately, and independently of that ordering: a read of the session id for a
log line or a span attribute can no longer abort the work it is describing.
Observational reads go through a total accessor that falls back to a placeholder
instead of throwing. Callers that genuinely cannot proceed without an id —
execution-owner routing, approval URLs — still get the strict accessor and its
throw, because a wrong id is worse there than a failure.

An alarm that can resolve no name from any source now logs that and exits
cleanly after releasing the runtime and dropping the alarm, rather than throwing
into an endless retry.

Testing

• packages/hosts/cloudflare/src/mcp/agent-session-durable-object.test.ts: four
  new cases over the existing ctx/storage harness — an alarm on a DO whose
  ctx.id.name is unset but whose durable record exists disposes the session
  the record names; the same shape extends a paused lease instead of dying on
  its log; an alarm with no name from any source completes and cleans up; an
  observational id read never throws. Full file: 23 passed.
• Mutation check: reverting the observational accessor to the raw PartyServer
  getter fails all four new cases and leaves the pre-existing 19 passing.
• packages/hosts/cloudflare MCP unit suites: 89 passed.
• e2e cloud/mcp-session-cold-init.test.ts and
  cloud/mcp-destroyed-session-envelope.test.ts: both pass, before and after.
• typecheck and lint green.

This is verified at the unit level by the declared exception for cases the dev
stack cannot produce: miniflare addresses every Durable Object by name, so an
alarm with an unset ctx.id.name cannot be provoked black-box.

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