refactor: reduce complexity of execute_add_reviewers in update_pr.rs

[github-actions]

What was complex

execute_add_reviewers in src/safeoutputs/update_pr.rs (line 648) had a cognitive complexity of 21 (threshold 15 scan; just below the default Clippy threshold of 25). The function combined three separate concerns inline inside a single for reviewer in reviewers loop:

1. VSSPS identity resolution — a 3-arm match on an HTTP response
2. Identity unwrapping — a 2-arm match with continue on None
3. PR reviewer PUT request — another 3-arm match on the HTTP response

The nesting depth drove the complexity score up: loop (1) → identity match (2) → PUT match (3), with each arm adding its own branches.

What changed

Introduced ReviewerAddResult enum — a small private enum with two variants:

• Added — reviewer was successfully added
• Failed(String) — short reason string (e.g. "identity not found", "HTTP 404", "request error")

Extracted resolve_and_add_reviewer — a module-level async fn that handles exactly one reviewer:

• Resolves the ADO identity via VSSPS
• PUTs the reviewer onto the PR
• Returns ReviewerAddResult

execute_add_reviewers simplified — the loop body collapses to a single match on ReviewerAddResult, accumulating added/failed lists. All the VSSPS and HTTP logic lives in the extracted helper.

Before / after complexity (at threshold = 15)

Function	Before	After
execute_add_reviewers	21	< 15 (no warning)
resolve_and_add_reviewer (new)	—	17

Behaviour preserved

• Success/failure messages identical (same format strings, same JSON output shape)
• All 1858 tests pass
• cargo clippy --all-targets --all-features is clean at the default threshold (25)

Warning

Firewall blocked 2 domains

The following domains were blocked by the firewall during workflow execution:

• spsprodeus21.vssps.visualstudio.com
• spsprodweu4.vssps.visualstudio.com

To allow these domains, add them to the network.allowed list in your workflow frontmatter:

network:
  allowed:
    - defaults
    - "spsprodeus21.vssps.visualstudio.com"
    - "spsprodweu4.vssps.visualstudio.com"

See Network Configuration for more information.

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