Improve inter-branch merge configuration errors

[missymessa]

Summary

• distinguish configuration download failures from JSON parsing failures
• fail the workflow for malformed JSON, invalid schema, and missing MergeToBranch
• preserve the successful no-op behavior when a valid configuration has no entry for the current branch
• clarify the no-configuration workflow message

Validation

• valid MSBuild configuration emits the expected outputs
• a valid configuration without a branch mapping exits successfully without outputs
• the historical malformed MSBuild configuration fails with the JSON parser's line and position
• a missing configuration file fails with its HTTP status
• invalid schema and missing MergeToBranch fail with targeted messages

Azure DevOps work item: https://dev.azure.com/dnceng/internal/_workitems/edit/10214

[Copilot]

Pull request overview

Improves inter-branch merge configuration error handling and clarifies no-op workflow messaging.

Changes:

• Separates download, JSON parsing, and schema validation failures.
• Validates configuration structure and required MergeToBranch.
• Clarifies the no-configuration workflow message.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Summary
.github/workflows/scripts/read-configuration.ps1	Adds configuration parsing and validation. A moderate issue remains: ContainsKey is called before the root-type guard, causing non-object JSON roots to produce method-invocation errors instead of targeted schema errors (lines 74 and 99).
.github/workflows/inter-branch-merge-base.yml	Updates the no-configuration workflow message.

Suppressed comments (1)

.github/workflows/scripts/read-configuration.ps1:100

• This presence check casts the value to a string, so schema-invalid JSON values such as a number, boolean, array, or object can pass as non-empty and be written to the workflow output. The merge step then treats that representation as a branch and can fail later or target the wrong branch. Require MergeToBranch itself to be a string before accepting it.

    if ($configuration.ContainsKey('MergeToBranch') -and
        ![string]::IsNullOrWhiteSpace([string]$configuration['MergeToBranch'])) {

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[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated no new comments.