Fix issue closed state colors to match GitHub.com behavior with state reason support

[Copilot]

This PR fixes the inconsistent coloring of closed issues in the PR webview to align with GitHub.com's visual design, including support for distinguishing between issues closed as "completed" vs "not planned".

Problem

Previously, both closed issues and closed pull requests displayed with red color in the extension's webview, which was inconsistent with GitHub.com where:

• Closed issues should display in purple (same as merged PRs) when completed
• Closed issues should display in grey when closed as "not planned"
• Closed pull requests should remain red

Solution

Updated the color logic to differentiate between closed issues and closed PRs using GitHub's stateReason field:

1. Added stateReason field to GraphQL queries to fetch issue closure reason (COMPLETED vs NOT_PLANNED)
2. Updated Issue interfaces in both interface.ts and views.ts to include the optional stateReason field
3. Enhanced getStatus() function in webviews/components/header.tsx to accept stateReason parameter and use appropriate colors:
   • Issues closed as "not planned": grey (draft color)
   • Issues closed as "completed": purple (merged color)
   • Closed PRs: red (unchanged)
4. Updated IssueModel to track and handle stateReason changes
5. Enhanced markdown icon colors in src/github/markdownUtils.ts to use grey (#6a737d) for "not planned" issues and purple (#8957e5) for "completed" issues

Visual Impact

• 🟣 Closed issues (completed): Display in purple (matching GitHub.com)
• 🔘 Closed issues (not planned): Display in grey (matching draft PR color)
• 🔴 Closed PRs: Continue to display in red (unchanged)
• 🟢 Open states: Remain green for both issues and PRs (unchanged)
• 🟣 Merged PRs: Continue to display in purple (unchanged)

This change makes the extension's visual language consistent with users' expectations from GitHub.com, improving the overall user experience and reducing confusion between different item states.

Fixes #7628.

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[alexr00]

@copilot Now, we are using the proper existing purple merged color for closed issues. The next step is to:

• Determine if an issue was closed as not planned or completed
• Use the exising grey "draft pr" color for issues closed as not planned issues.

[Copilot]

@copilot Now, we are using the proper existing purple merged color for closed issues. The next step is to:

• Determine if an issue was closed as not planned or completed
• Use the exising grey "draft ...

I've implemented the stateReason functionality to distinguish between issues closed as "not planned" vs "completed":

Changes made in commit 1a22a03:

• Added stateReason field to GraphQL queries to fetch closure reason
• Updated Issue interfaces to include the stateReason field
• Modified getStatus() function to use grey "draft" color for issues closed as NOT_PLANNED
• Enhanced markdown icon colors to use grey (#6a737d) for not planned issues

Color behavior:

• 🔘 Issues closed as "not planned": Grey (using existing draft PR color)
• 🟣 Issues closed as "completed": Purple (using merged PR color)
• 🔴 Closed PRs: Red (unchanged)

The implementation leverages GitHub's stateReason field which can be NOT_PLANNED, COMPLETED, or REOPENED for issues.