fix(eve): degrade missing attachment bytes on resume instead of failing the turn

[serhiizghama]

Fixes #276. Resuming a durable session whose history contains an inbound file attachment fatally fails when the staging sandbox has been torn down between turns — an ephemeral backend means the bytes behind a historical eve-sandbox: ref are gone, and hydrateMessageContent threw on the missing read, so the turn ended in session.failed and the bot never replied.

Hydration now degrades a missing ref to a text part ("Attached file <path> (<type>) is no longer available in this session.") and logs a warning, instead of throwing. The run continues, and the model is told the attachment is unreachable rather than chasing a path that no longer resolves. I gave it a distinct message from the normal non-inlinable text reference on purpose — on resume the path really is gone, so pointing the model at it would just invite a failed read.

This only affects the resume case where the bytes are already missing; a freshly staged attachment in the current turn still hydrates as before.

Tests: rewrote the existing "missing file" case to assert the degrade (the leading text survives, the file ref becomes the unavailability notice, no file part reaches the model) and added a regression that stages an image into one sandbox and hydrates the resulting ref against a fresh one — the same shape as a resume after teardown — and asserts the turn survives. Reverting the fix fails both. Full attachment-staging integration + unit suites green, typecheck/oxlint/oxfmt/guard:invariants clean, changeset (patch).

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