fix(compile): address PR review feedback on migration framework

Four findings from the Rust PR Reviewer bot:

1. check_pipeline's bail message had a redundant "error:" prefix that
   produced "Error: error: ..." once anyhow's top-level handler added
   its own "Error:" wrapper. Drop the prefix and reformat the hint
   onto an indented continuation line.

2. compile_pipeline_inner did a redundant serde_yaml::from_value
   round-trip just to satisfy ParsedSource.front_matter, even though
   perform_source_rewrite_if_needed never reads that field. Refactor
   the helper to take the four primitive fields it actually uses
   (mapping, body_raw, source_sha256, migrations) instead of the full
   struct. reconstruct_source likewise takes individual fields now.

3. Misleading comment in tests/migration_tests.rs claimed a
   "thread-local counter" was used; only a nanosecond timestamp was.
   Replaced rand_suffix() with timestamp + AtomicU64 monotonic seq so
   parallel tests scheduled in the same nanosecond always get
   distinct dirs.

4. Leading whitespace before the opening `---` was silently stripped
   on migration rewrite. parse_markdown_detailed already tolerates it
   on read; capture it as a new ParsedSource.leading_whitespace field
   and emit it from reconstruct_source so byte-faithful preservation
   extends to whitespace prefixes (BOM-stripped editor blank lines,
   etc.).

All 1369 tests still pass.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: jamesadevine

scripts/ado-script/dist/gate/index.js

@@ -0,0 +1 @@
+{"name":"azure-devops-node-api","description":"Node client for Azure DevOps and TFS REST APIs","version":"14.1.0","main":"./WebApi.js","types":"./WebApi.d.ts","scripts":{"build":"node make.js build","samples":"node make.js samples","test":"node make.js test","units":"node make.js units"},"repository":{"type":"git","url":"https://github.com/Microsoft/azure-devops-node-api"},"author":"Microsoft Corporation","contributors":["Bryan MacFarlane <bryanmac@microsoft.com>","Daniel McCormick<daniel.mccormick@microsoft.com>","Scott Dallamura <scdallam@microsoft.com>","Stephen Franceschelli<stephen.franceschelli@microsoft.com>","Teddy Ward <t-edward@microsoft.com>"],"license":"MIT","engines":{"node":">= 16.0.0"},"dependencies":{"tunnel":"0.0.6","typed-rest-client":"2.1.0"},"devDependencies":{"@types/glob":"5.0.35","@types/minimatch":"3.0.3","@types/mocha":"^2.2.44","@types/node":"16.11.7","@types/shelljs":"0.8.5","mocha":"^10.4.0","nock":"9.6.1","shelljs":"^0.8.5","typescript":"4.0.2"},"type":"module"}

scripts/ado-script/dist/gate/package.json

@@ -0,0 +1,3 @@
+{
+  "type": "module"
+}