Fix: Search entire attribute list for {OriginalFormat}

[VanshikaNavya]

The TryGetOriginalFormatFromAttributes method only checked the last position for the {OriginalFormat} attribute. This fails when using Microsoft.Extensions.Telemetry.Abstractions logging source generator, which places the attribute at the first position.

Fixes #
This resolves the issue where LogRecord.Body contained the formatted message instead of the template when using the MS Telemetry Abstractions generator.

Design discussion issue #
#5983

Changes

• Modified the TryGetOriginalFormatFromAttributes to first check the last position (most common - built-in MS logger)
• If not found, traverses from the beginning through remaining positions
• Ensures O(1) performance for 99% of cases while maintaining correctness for all scenarios
• Handles edge cases where {OriginalFormat} appears at any position in the attribute list

Merge requirement checklist

• CONTRIBUTING guidelines followed (license requirements, nullable enabled, static analysis, etc.)
• Unit tests added/updated
• Appropriate CHANGELOG.md files updated for non-trivial changes
• Changes in public API reviewed (if applicable)

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: VanshikaNavya / name: Vanshika Navya (18521a9)

[martincostello]

Could you add a test that would fail without the fix so we know it is indeed fixed?

[martincostello]

Maybe this could be simplified by just using having the for loop, but going backwards?

for (int i = attributes.Count - 1; i > -1; i--)
{
    var attribute = attributes[i];
    if (attribute.Key == "{OriginalFormat}" && attribute.Value is string value)
    {
        originalFormat = value;
        return true;
    }
}

[VanshikaNavya]

Yes, the backwards loop is cleaner and still checks the most common position (last index) first. Updated!

[martincostello]

Suggested change

	{
	{