fix(schema): ensure experiment_id is string in series representation

[michael-johnston]

The experiment_id field in ValidMeasurementResult.series_representation and Entity.experiment_series was being set to the ExperimentReference instance.

This rendered as the id when outputting as CSV. However, when outputting as json it was the pydantic JSON dump.

The field properly should just be the string id c.f. entity "identifier" field. This commit makes this the case.

[DRL-NextGen]

Checks Summary

Last run: 2026-03-18T10:32:24.941Z

Code Risk Analyzer vulnerability scan found 1 vulnerabilities:

Severity	Identifier	Package	Details	Fix
◻ Unknown	CVE-2026-25048	xgrammar	xgrammar vulnerable to DoS via multi-layer nesting GHSA-7rgv-gqhr-fxg3 xgrammar:0.1.29->vllm:0.17.1	0.1.32

Mend Unified Agent vulnerability scan found 1 vulnerabilities:

Severity	Identifier	Package	Details	Fix
🔺 High	CVE-2026-30922	pyasn1-0.6.2-py3-none-any.whl	pyasn1 is a generic ASN.1 library for Python. Prior to 0.6.3, the "pyasn1" library is vulnerable to ... pyasn1 is a generic ASN.1 library for Python. Prior to 0.6.3, the "pyasn1" library is vulnerable to a Denial of Service (DoS) attack caused by uncontrolled recursion when decoding ASN.1 data with deeply nested structures. An attacker can supply a crafted payload containing thousands of nested "SEQUENCE" ("0x30") or "SET" ("0x31") tags with "Indefinite Length" ("0x80") markers. This forces the decoder to recursively call itself until the Python interpreter crashes with a "RecursionError" or consumes all available memory (OOM), crashing the host application. This is a distinct vulnerability from CVE-2026-23490 (which addressed integer overflows in OID decoding). The fix for CVE-2026-23490 ("MAX_OID_ARC_CONTINUATION_OCTETS") does not mitigate this recursion issue. Version 0.6.3 fixes this specific issue.	Upgrade to version https://github.com/pyasn1/pyasn1.git - v0.6.3,pyasn1 - 0.6.3

[AlessandroPomponio]

Doesn't str(e) work as well? I'd avoid using an fstring if possible because it's more likely for someone to end up adding a space or other whitespace inside an fstring and for that to break checks

[michael-johnston]

Doesn't str(e) work as well? I'd avoid using an fstring if possible because it's more likely for someone to end up adding a space or other whitespace inside an fstring and for that to break checks

Sure

[michael-johnston]

Changes made.

[AlessandroPomponio]

LGTM thanks

[michael-johnston]

@AlessandroPomponio I simplified one change.

[AlessandroPomponio]

LGTM thanks