Document DOTNET_Thread_DefaultStackSize environment variable for .NET 9

[Copilot]

Adds documentation for the new DOTNET_Thread_DefaultStackSize environment variable introduced in .NET 9, which configures the default stack size (in bytes) for VM threads.

Changes

• Added new section "Default stack size for new threads" to threading.md
• Included configuration table with environment variable details
• Added usage examples for Windows, Linux, and macOS
• Added C# code reference showing thread creation behavior
• Noted that threads created with explicit stack size remain unaffected
• Fixed markdown linting issues (blank lines around headings and code blocks)

Example Usage

export DOTNET_Thread_DefaultStackSize=2097152  # 2 MB
dotnet myapp.dll

This setting affects threads created via System.Threading.Thread and similar APIs that don't explicitly specify stack size. If unset, the system or runtime default applies.

Original prompt

Update docs/core/runtime-config/threading.md to include documentation for the new environment variable DOTNET_Thread_DefaultStackSize=val, which sets the stack size, in bytes, for new VM threads created with the default stack size. Add a section describing what it does, a configuration table, usage examples for several platforms, and a note that threads created with an explicit stack size are unaffected. Include this in the appropriate section following the style of the current threading configuration documentation. Make sure to add the frontmatter ai-usage: ai-assisted.

Example table entry:

	Setting name	Values	Version introduced
Environment variable	DOTNET_Thread_DefaultStackSize	An integer value (bytes). Example: 1048576 for 1 MB	.NET 9

This pull request was created as a result of the following prompt from Copilot chat.

Update docs/core/runtime-config/threading.md to include documentation for the new environment variable DOTNET_Thread_DefaultStackSize=val, which sets the stack size, in bytes, for new VM threads created with the default stack size. Add a section describing what it does, a configuration table, usage examples for several platforms, and a note that threads created with an explicit stack size are unaffected. Include this in the appropriate section following the style of the current threading configuration documentation. Make sure to add the frontmatter ai-usage: ai-assisted.

Example table entry:

	Setting name	Values	Version introduced
Environment variable	DOTNET_Thread_DefaultStackSize	An integer value (bytes). Example: 1048576 for 1 MB	.NET 9

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