filer.conf: merge fsync on presence like worm (#11953)

* filer.conf: merge fsync on presence like worm

fsync is a pure preference, so OR-merging it meant a nested rule could
never turn a parent's fsync off. Make PathConf.fsync optional and merge
it most-specific-wins, matching worm: an unset rule inherits, an explicit
value overrides in either direction. readOnly and disableChunkDeletion
stay OR'ed — they are fail-closed locks quota enforcement depends on.

Pre-version-2 configs were written with EmitUnpopulated, so every rule
carries an explicit fsync=false that meant nothing; it is dropped to
unset on load so it does not read as an opt-out.

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Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

* filer.conf: test fsync override survives a save/reload round trip

---------

Co-authored-by: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

other/java/client/src/main/proto/filer.proto

@@ -771,7 +771,8 @@ message FilerConf {
         string replication = 3;
         string ttl = 4;
         string disk_type = 5;
-        bool fsync = 6;
+        // unset inherits from the enclosing path rule, set overrides it
+        optional bool fsync = 6;
         uint32 volume_growth_count = 7;
         bool read_only = 8;
         string data_center = 9;