fix: handle invalid JSON escape sequences in partial-json-parser

[JosephDoUrden]

Summary

Fix partialParse crashing on invalid JSON escape sequences (e.g. \H, \M from PHP namespaces like App\Http\Middleware) by repairing them in the generate() function before JSON.parse().

Fixes #986

Problem

When a model emits tool call JSON containing unescaped backslashes (e.g. PHP namespaces), partialParse throws Bad escaped character in JSON, crashing MessageStream during input_json_delta accumulation and killing the entire streaming response.

The tokeniser correctly captures escape sequences like \H, but generate() outputs them verbatim into a JSON string. JSON.parse() then rejects them since only \" \\ \/ \b \f \n \r \t \uXXXX are valid JSON escapes.

Fix

In generate(), sanitise string token values by doubling backslashes before invalid escape characters:

token.value.replace(/\\([^"\\\/bfnrtu])/g, '\\\\$1')

This turns \H into \\H, \M into \\M, etc., producing valid JSON.

Changes

• src/_vendor/partial-json-parser/parser.ts: Sanitise invalid escapes in generate()
• tests/lib/partial-json.test.ts: Added tests for invalid escape sequences and mixed valid/invalid escapes

Test plan

• All 19 partial-json tests pass (17 existing + 2 new)
• New test: PHP namespace backslashes (App\Http\Middleware) parse correctly
• New test: Mixed valid (\n) and invalid (\H) escapes in same string