Fix compatibility issue where coll_t::_str retained stale values after
decoding v1/v2 format blobs, causing confusing debug output and tool
messages.

The _str field was not recalculated when decoding older formats:

• v1 format (pre-Ceph v0.21, before commit tchaikov@a108774)
• v2 format (meta/regular PGs)
• v3 format always includes _str (temp PGs)

This primarily affected debugging scenarios since _str is only used for
log messages and BlueStore low-level tool output. The issue went
undetected because existing tests reused struct instances, preserving
field values across encode/decode cycles.

An upcoming test change will allocate fresh instances for each decode, which would expose these stale values. Since _str can be derived from always-encoded fields, recalculate it during decode() when missing from the encoded data.

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