Wrap the pre-session v2 policy change with timeout

[IntelCaisui]

Wrap the pre-session v2 policy change with a timeout to prevent potential
hang during pre migration.
Fix problem 1 of issue: #603

[Copilot]

Pull Request Overview

This PR adds timeout protection for the pre-session v2 policy exchange to prevent potential hangs during migration, addressing issue #603. The changes refactor timeout constant definitions to be scoped closer to their usage points.

Key changes:

• Wraps the pre-session policy exchange with a 60-second timeout
• Moves timeout constant definitions from global scope to function scope
• Consolidates timeout constant declarations for better code organization

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
src/migtd/src/spdm/mod.rs	Removes global SPDM_TIMEOUT constant definition
src/migtd/src/migration/session.rs	Adds timeout wrapper for policy exchange and moves timeout constants to local scope

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[Copilot]

The constant PRE_SESSION_TIMEOUT is defined inside the function but before a feature gate. Consider moving it inside the #[cfg(feature = "policy_v2")] block since it's only used within that feature context, similar to how SPDM_TIMEOUT is placed inside its feature gate at line 994-995.

Suggested change

	const PRE_SESSION_TIMEOUT: Duration = Duration::from_secs(60); // 60 seconds
	#[cfg(feature = "policy_v2")]
	let remote_policy = Box::pin(with_timeout(
	PRE_SESSION_TIMEOUT,
	pre_session_data_exchange(&mut transport),
	))
	.await??;
	{
	const PRE_SESSION_TIMEOUT: Duration = Duration::from_secs(60); // 60 seconds
	let remote_policy = Box::pin(with_timeout(
	PRE_SESSION_TIMEOUT,
	pre_session_data_exchange(&mut transport),
	))
	.await??;
	}

[IntelCaisui]

remote_policy will be used later, connot warp it with {} false suggestion.
But later SPDM_TIMEOUT could adopt this style. Updated.