Centralize JSON escaping for pipeline variables at manager level

[Copilot]

When syncing environment variables to remote pipelines (GitHub Actions/Azure DevOps), values containing JSON-like content (e.g., ["api://guid"]) were being incorrectly parsed as JSON arrays instead of strings, causing Bicep parameter failures.

Changes

cli/azd/pkg/pipeline/pipeline.go:

• Added escapeValuesForPipeline() to apply JSON escaping using json.Marshal(), stripping outer quotes
• Modified mergeProjectVariablesAndSecrets() to escape both variables and secrets before returning
• Moves escaping logic from individual providers (GitHub/AzDo) to centralized pipeline manager per reviewer feedback

cli/azd/pkg/pipeline/pipeline_escape_test.go:

• Added test coverage for JSON-like strings, simple values, and edge cases

Example

Before:

// In .azure/envName/config.json
{"ValidAudiences": "[\"api://guid\"]"}

// Sent to pipeline (incorrectly parsed as array)
["api://guid"]

After:

// In .azure/envName/config.json  
{"ValidAudiences": "[\"api://guid\"]"}

// Sent to pipeline (correctly escaped as string)
[\"api://guid\"]

The escaping uses standard JSON marshaling to handle all special characters (quotes, backslashes, brackets), ensuring pipeline providers treat values as strings regardless of content.

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