Add default value support for interpolation expressions

[Copilot]

Implements ${key:defaultValue} syntax where defaultValue is used when key is not found in value sources.

Changes

Core interpolation logic (StringSearchInterpolator, FixedStringSearchInterpolator, MultiDelimiterStringSearchInterpolator):

• Parse expressions for : separator to extract key and default value
• First attempt resolution of full expression (preserves backward compatibility for keys containing colons)
• If unresolved and colon present, split and resolve key portion only
• Use default value when key lookup fails
• Added findMatchingEndExpr() to correctly handle nested expressions like ${missing:${fallback}}

Backward compatibility:

• Keys containing colons (e.g., url:key) continue to work if they exist in value sources
• Empty key syntax (e.g., ${:default}) remains unresolved

Usage

Properties p = new Properties();
p.setProperty("version", "1.0.0");
p.setProperty("fallback.url", "http://example.com");

StringSearchInterpolator interpolator = new StringSearchInterpolator();
interpolator.addValueSource(new PropertiesBasedValueSource(p));

// Uses existing value
interpolator.interpolate("${version:SNAPSHOT}");  // → "1.0.0"

// Uses default when missing
interpolator.interpolate("${missing:default}");  // → "default"

// Nested expressions in defaults
interpolator.interpolate("${url:${fallback.url}}");  // → "http://example.com"

// Colons in defaults (URLs, paths)
interpolator.interpolate("${repo:https://github.com/example/repo}");  // → "https://github.com/example/repo"

Original prompt

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This section details on the original issue you should resolve

<issue_title>Default Values in Interpolation</issue_title>
<issue_description>The current interpolation logic does not seem to support default values, i.e. ${primary.value:fallback}, instead treating the entire primary.value:fallback as the property key. This may be conflating resource filtering with actual maven property expressions, but I do still think the ability to have default values would be of value. Is there another way to achieve this currently?</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Default Values in Interpolation #73

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[cstamas]

I am unsure... Maven 4 does not use this anymore, as it has own interpolation service. Basically this component is actively used in Maven 3 realm only, and as such, unsure do we want to add new features to it?

[slachiewicz]

Good point, let's reject this feature then.