Add ActivityTraceFlags.RandomTraceId flag

[Kielek]

Fixes #124509

Brings support for RandomTraceId flag known from W3C context propagation level 2.
More details in linked issue.

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[Copilot]

Pull request overview

Adds support for the W3C Trace Context Level 2 random trace-flags bit by introducing ActivityTraceFlags.RandomTraceId and ensuring Activities created with the default trace-id generator set/propagate this flag.

Changes:

• Add ActivityTraceFlags.RandomTraceId to the public API surface.
• Add Activity.RandomizedTraceId convenience property and set the flag when the default random trace-id generator is used.
• Expand ActivityTests coverage for parsing/propagating the new flag and validating default generation behavior.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
src/libraries/System.Diagnostics.DiagnosticSource/src/System/Diagnostics/Activity.cs	Introduces RandomizedTraceId, sets RandomTraceId when using the default generator, and adds the new enum flag.
src/libraries/System.Diagnostics.DiagnosticSource/ref/System.Diagnostics.DiagnosticSourceActivity.cs	Updates reference assembly API surface for the new property and enum member.
src/libraries/System.Diagnostics.DiagnosticSource/tests/ActivityTests.cs	Adds/extends unit tests for RandomTraceId parsing, propagation, and default generation behavior.

Comments suppressed due to low confidence (1)

src/libraries/System.Diagnostics.DiagnosticSource/src/System/Diagnostics/Activity.cs:950

• The XML documentation for RandomizedTraceId/Recorded refers to W3CIdFlags.*, but there is no public (or internal) W3CIdFlags type; the flag enum is ActivityTraceFlags. Update the docs to reference ActivityTraceFlags.RandomTraceId / ActivityTraceFlags.Recorded (or describe the W3C trace-flags field generically) to avoid misleading API docs.

        /// <summary>
        /// True if the W3CIdFlags.RandomTraceId flag is set.
        /// </summary>
        public bool RandomizedTraceId { get => (ActivityTraceFlags & ActivityTraceFlags.RandomTraceId) != 0; }

        /// <summary>
        /// True if the W3CIdFlags.Recorded flag is set.
        /// </summary>
        public bool Recorded { get => (ActivityTraceFlags & ActivityTraceFlags.Recorded) != 0; }

[tarekgh]

@Kielek thanks for submitting it. We need to design review it before we proceed with the changes. I have converted this PR to be draft for now till we do that. If you like we can temporary close the PR and then reactive it back when ready.