fix(web-core): JSON-stringify objects/arrays in formatString per spec

[BBC6BAE9]

Description

FormatStringImplementation used Array.prototype.join('') which relied on JS default String(value) coercion, producing [object Object] for objects and flattened comma-separated values for arrays. The a2ui_protocol.md §"Type conversion" requires objects/arrays to be stringified as JSON.

Changes:

• Add coerceToString helper that implements the spec's type conversion rules: null/undefined → "", objects/arrays → JSON.stringify(), scalars → String()
• Update FormatStringImplementation to use coerceToString
• Add 5 unit tests covering object, array, nested array, array-with-null, and null interpolation
• Update CHANGELOG

Reproduction (from issue):

Template	Spec says	web_core before fix
"Tags: ${/tags}"	Tags: ["swift","ios"]	Tags: swift,ios
"User: ${/user}"	User: {"name":"Alice","age":30}	User: [object Object]
"M = ${/matrix}"	M = [[1,2],[3,4]]	M = 1,2,3,4
"V = ${/vals}"	V = [1,null,3]	V = 1,,3

Fixes #1386

Pre-launch Checklist

• I signed the CLA.
• I read the Contributors Guide.
• I read the Style Guide.
• I have added updates to the CHANGELOG.
• I updated/added relevant documentation.
• My code changes (if any) have tests.
• If my branch is on fork, I have verified that scripts/e2e_test.sh passes.

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[gemini-code-assist]

Code Review

This pull request updates the formatString function to align with the A2UI protocol by JSON-stringifying objects and arrays and converting null/undefined values to empty strings. A potential issue was identified where JSON.stringify could cause the UI to crash if it encounters circular references or BigInt values; a more robust implementation with error handling and a fallback was suggested.