fix(harness): mark optional FilesystemTool params as required=false

[wzq-xzwj]

Summary

Several @ToolParam parameters on FilesystemTool are documented as optional
(their descriptions say "Default: ...", "Optional ...") and the underlying
AbstractFilesystem implementation already handles missing / null / default
values gracefully. However, they are not annotated with required = false.

Because ToolParam.required() defaults to true, these parameters end up in the
generated JSON schema as required, contradicting both their descriptions and
the implementation.

Affected parameters:

Tool	Param	Description says	Was in schema
read_file	offset	"Default: 0 (from beginning)"	required
read_file	limit	"Default: 0 (all lines)"	required
grep_files	path	dir/file to search	required
grep_files	glob	"Optional file glob filter"	required
glob_files	path	base dir to search from	required

Why this matters

Models calling these tools believe they must supply every parameter. In practice
this pushes the model to always pass a glob filter to grep_files (narrowing
results unintentionally) or an explicit offset/limit to read_file, and often
leads models to prefer shell commands over the built-in tools. The mismatch
between the human-readable description ("Optional") and the schema ("required")
is confusing for both models and integrators.

Note: edit_file's replace_all is already correctly annotated with
required = false, which shows the intended pattern — the other optional
parameters were simply missed.

Implementation is already optional-safe

The backing implementation already tolerates missing values, so this is a
schema-only correction with no behavioral risk:

• LocalFilesystem.grep(...): resolvePath(runtimeContext, path != null ? path : ".")
• LocalFilesystem.glob(...): handles path == null → falls back to cwd
• ripgrepSearch / javaSearch: if (includeGlob != null && !includeGlob.isBlank())
• LocalFilesystem.read(...): int startIdx = Math.max(0, offset), limit > 0 ? ... : all

Change

Add required = false to the five parameters listed above. No logic changes.

Testing

• Schema generation now marks these parameters as optional.
• Existing behavior unchanged (implementation already handled absent values).

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