add-pr-comment: render inline threads as applyable single/multi-line suggestions

## Diagnostic Summary

- **Pipeline**: AITestAgent Issue Fixer (an `ado-aw` agentic workflow, `devdiv/DevDiv`)
- **Definition ID**: Unknown
- **Build ID**: Unknown — the defect is in safe-output rendering, not a build failure; reproduced deterministically with `ado-aw` `execute` locally (v0.37.0 + patched build)
- **Run URL (production artifact)**: PR #750251, thread #9557714 — `https://devdiv.visualstudio.com/DefaultCollection/DevDiv/_git/AITestAgent/pullRequest/750251?discussionId=9557714`
- **Result**: Partially succeeded — the `add-pr-comment` POST returns HTTP 200, but the persisted thread renders as a **non-applyable** suggestion
- **Failing stage/job/step**: Stage 3 (SafeOutputs) → SafeOutputs → `add-pr-comment`
- **First failed timeline record**: N/A (no hard failure; defective output)
- **Suspected root cause**: `add-pr-comment` anchors the inline thread with a zero-width range and HTML-escapes `<`/`>` inside fenced **and** inline code
- **Confidence**: High

## Evidence

### Relevant log excerpts
Read back from the ADO REST API for the production thread (`threads/9557714`):
- `threadContext.rightFileStart = { line: 90, offset: 1 }`, `rightFileEnd = { line: 90, offset: 1 }` — a **zero-width** anchor. ADO treats it as an insertion point, so "Apply" prepends the suggested line to the original (duplicated/concatenated line).
- The ```` ```suggestion ```` body and the prose inline-code spans contain `<`/`>` instead of literal `<`/`>`, so applying writes HTML entities into the file.

### Timeline observations
- The safe output executed "successfully" (HTTP 200); there is no build-level failure. The defect is entirely in the rendered/persisted thread shape.

### Changes since last successful build
- Not a user regression — latent in the runtime's anchor + escaping logic. Present across all `add-pr-comment` inline suggestions.

## Environment

- **Agent source file**: AITestAgent Issue Fixer workflow `.md` (in the AITestAgent ADO repo) — path Unknown
- **Compiled pipeline YAML**: Unknown
- **Compilation in sync**: Unknown
- **ado-aw version**: v0.37.0 (reproduced locally; also on a patched build of `main`)
- **AWF version**: Unknown
- **MCPG version**: Unknown
- **Agent pool**: ubuntu-22.04
- **Engine/model**: copilot / claude-opus-4.7
- **Relevant MCP servers**: SafeOutputs

## Analysis

- **Stage classification**: Stage 3 (SafeOutputs)
- **Why this stage failed**: `add-pr-comment` builds a `threadContext` whose right-side end anchor is offset `1` (zero-width), and runs the whole comment body — including the ```` ```suggestion ```` fence and inline `` `code` `` spans — through `sanitize()` → `escape_html_tags`, which escapes every `<`/`>`. ADO renders fenced/inline code verbatim and never interprets HTML there, so the escaping is both unnecessary and corrupting; the zero-width anchor makes "Apply" non-idempotent.

## Root Cause

- **Root cause**: three independent defects, all reproduced and fixed:
1. `src/safeoutputs/add_pr_comment.rs` — inline thread anchored with a zero-width right-side range (`offset 1`) instead of spanning the whole target line(s). Fix: anchor `rightFileEnd` to `(UTF-16 length of the last line) + 1`.
2. `src/sanitize.rs` — `escape_html_tags` escaped `<`/`>` inside ```` ``` ````/`~~~` fences. Fix: fence-aware skip.
3. `src/sanitize.rs` — same escaper mangled inline `` `code` `` spans in prose. Fix: inline-code-aware skip with a security guard for unmatched backticks.
- **Category**: Runtime regression
- **Ruled-out causes**: network/AWF (HTTP 200), write-token/permissions (thread created), pool/image, threat-analysis block — none involved; defect reproduces deterministically offline.