perf(fs): probe tile extensions with File.Exists instead of directory glob (MAPCO-11364 P14)

[shimoncohen]

P14 of MAPCO-11364 (LOGIC-3 I/O follow-up). Stacked on #257 (logic-3-io-async) — retarget to master after #257 merges.

Change

FileClient.GetTilePath (used by GetTile and TileExists) globbed the directory with EnumerateFiles(path, "{z}/{x}/{y}.*") on every call. Replaced with direct File.Exists probes of the supported extensions — jpeg then png.

• File.Exists returns false for both a missing tile and a missing directory, so the DirectoryNotFoundException catch is no longer needed and is removed.
• The probe set is sourced from Enum.GetValues<TileFormat>() — the TileFormat enum is the single source of truth, so a new format is covered automatically (no hardcoded array).

Cross-cutting: TileFormat enum reordered

TileFormat is reordered Jpeg, Png (was Png, Jpeg) so declaration order — which is now the probe order via Enum.GetValues — keeps jpeg first, matching the S3 client's historical probe order.

• Verified no int-value coupling on TileFormat: no (int) casts, no default(TileFormat) reliance, no ordinal indexing. All 11 usages are explicit .Jpeg/.Png. The reorder is behavior-preserving.
• Full MergerLogic suite (1117) and MergerService suite (23) both green after the reorder.

Follow-up

The extension string is still derived from format.ToString().ToLower() (here and in PathUtils), which only coincidentally equals the [EnumMember] value. Tracked as MAPCO-11369 (make the EnumMember value the source of truth for the extension).

Tests

• GetTile / TileExists now mock Path.Combine + File.Exists via a shared SetupTilePathProbe helper (png setups omitted when jpeg is found, per strict mocks).
• TileExistsReturnFalseWhenDirectoryDontExist removed — File.Exists never throws on a missing directory, so the scenario collapses into the normal "not found → false" case.
• FileClient suite: 10 passed, 0 failed.

Scope

Only FileClient (+ the TileFormat reorder). Sibling 11364 items (P5r, P11, P10r, P15, P4) are separate PRs.

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