* server : preserve context checkpoints across slot save/restore

Append the checkpoints after the packed server_tokens payload added in #26640
and count them in n_written / n_read, so a restored slot can still roll back to
a checkpoint instead of re-processing the whole prompt.

* server : drop draft checkpoint data that does not match the draft context

Restoring a slot saved with a different draft KV cache type aborted in
load_dft(). Test-load one draft checkpoint on restore and drop the draft
data if it does not fit, instead of crashing. Adds a regression test.

Co-authored-by: Igor Okulist <okigan@gmail.com>

* server : harden the checkpoint appendix of slot save files

Bound each blob size by the bytes left in the file before allocating, open the
file with UTF-8 paths on Windows like the llama state payload, fall back to full
prompt re-processing when a checkpoint restored from a slot file fails to load,
and replace the 1024 count cap by keeping the last n_ctx_checkpoints while reading.

* server : report an incomplete checkpoint appendix as a failed slot save

Return an error to the client when the appendix cannot be written, like a
failed payload write, and make the oversized-blob test declare a size that
cannot be allocated, so an unbounded allocation fails the test.

* server : reject an empty target state in the checkpoint appendix

A saved checkpoint always holds a target state, an empty blob would roll back
without restoring anything. Also log with the slot id, and load the draft test
model from the HF cache instead of a second download.

* common : return bool from checkpoint load_tgt / load_dft

A checkpoint restored from a slot file falls back to full prompt re-processing
when it fails to load, a checkpoint created in memory still aborts.

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Co-authored-by: Igor Okulist <okigan@gmail.com>