fix: unwrap the json fence when streaming in schema-less JSON mode

[Abhishek-B-R]

doGenerate strips the ```json fence Interfaze puts around json_object output, but doStream didn't. so streamText with Output.json() never produced a partial object and await output threw NoObjectGeneratedError, while generateText with the same arguments worked fine. that's the combination the run tasks docs show for the AI SDK.

this adds a small streaming version of stripJsonFence. it only holds text back while it can't yet tell whether a fence is opening or closing (the first few chars, and trailing whitespace or backticks), so partial JSON still streams as it arrives. it only runs in the same mode doGenerate unwraps (json, no schema).

tests: a fixture with the fence split across chunks, doStream in both modes, an end to end streamText + Output.json() case, and unit tests for the filter. the two new stream tests fail on main; all 86 pass with the fix. type-check, prettier, build and check:pkg are clean.

[Abhishek-B-R]

@Khurdhula-Harshavardhan friendly bump: this brings doStream in line with doGenerate by stripping the ```json fence in schema-less JSON mode, so streamText with Output.json() yields partial objects instead of ending in NoObjectGeneratedError. It merges cleanly with main, and the one workflow only needs maintainer approval to run. Happy to adjust anything if the approach doesn't fit.

[Abhishek-B-R]

@Khurdhula-Harshavardhan checking back once more: this is still the single commit 9337632 and merges cleanly with main; the one workflow just needs maintainer approval to run on this fork PR. If you'd prefer the fence handling in doStream done differently, I'm happy to rework it.