Update ValueOverflowThreshold

[badrishc]

ValueOverflowThreshold (Options and GarnetServerOptions) changes from int to a memory-size string (e.g. '4k', '1m'); default is '4k'.

All byte-level validation lives in GarnetServerOptions.ValueOverflowThresholdBytes() (single source of truth, no attribute-level duplication):

• Parsed via ServerOptions.TryParseSize.
• Range enforced as [64 B, 256 MB], matching Tsavorite's MaxInlineValueSize bounds (1 << LogSettings.kLowestMaxInlineSizeBits .. 1 << (LogSettings.kMaxStringSizeBits - 1)).
• Cross-property check: the value (after rounding down to previous power of 2) must be strictly less than the (rounded) PageSize, so an inline value of this size plus per-record overhead can fit on a page.

ServerOptions.PageSizeBits() enforces a minimum PageSize of 512 bytes (MinPageSizeBytes constant). At Garnet's defaults (MaxInlineKeySize = 128 B Tsavorite default, single-byte namespace), the worst-case inline record at PageSize = 512 B with the cross-check applied (effective MaxInlineValueSize <= 256 B) plus the 64 B page header is ~490 B; 512 B is the smallest power-of-2 page size that accommodates this and prevents Tsavorite's 'Entry does not fit on page' exception.

Help text on PageSize and ReadCachePageSize updated to mention the 512 B minimum.

Side fix: tighten MemorySizeValidationAttribute regex from
'^\d+([K|k|M|m|G|g][B|b]{0,1})?$' to '^\d+([KkMmGg][Bb]?)?$'.
The old form treated '|' as a literal inside the character class and accepted strings like '4|'.

defaults.conf updated: ValueOverflowThreshold value '4096' -> '"4k"', and PageSize / ReadCachePageSize comments note the 512 B minimum.

Tests added in GarnetServerConfigTests:

• ValueOverflowThresholdParsing (default + valid CLI/JSON values).
• ValueOverflowThresholdValidation (invalid format + out-of-range rejected at server-options consumption time).
• ValueOverflowThresholdMustFitOnPage (cross-check vs PageSize).
• MinimumPageSize (PageSize below 512 B rejected).

Breaking change: existing JSON configs that used a numeric ValueOverflowThreshold (e.g. 4096) must switch to a string ('4k' or '4096').

[Copilot]

Pull request overview

This PR changes ValueOverflowThreshold from an integer byte count to a memory-size string, centralizes byte-level validation in GarnetServerOptions, and updates related configuration defaults/help text and tests.

Changes:

• Parses and validates ValueOverflowThreshold as a memory-size string with bounds and page-fit checks.
• Adds a 512-byte minimum for main-log page size.
• Updates defaults/help text and adds configuration parsing/validation tests.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
libs/server/Servers/GarnetServerOptions.cs	Converts threshold handling to string parsing and validation before passing to Tsavorite settings.
libs/server/Servers/ServerOptions.cs	Adds a minimum effective main-log page size check.
libs/host/Configuration/Options.cs	Updates CLI option type/help text for threshold and page settings.
libs/host/Configuration/OptionsValidators.cs	Tightens memory-size regex validation.
libs/host/defaults.conf	Updates default threshold value and config comments.
test/Garnet.test/GarnetServerConfigTests.cs	Adds tests for threshold parsing, validation, page-fit checks, and page minimums.