Fix: make formatError's fallback stringification total

[AmaadMartin]

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Link to Issue or Description of Change

1. Link to an existing issue (if applicable):

No issue. core/src/utils/error_utils.ts landed in google#527.

2. Or, if no issue exists, describe the change:

Problem: The formatError doc comment promises that it never throws, but the function had no guard. String(err) in baseMessage throws on a null-prototype object and on any value whose toString throws, and extractHttpDetails throws on a value with a throwing status, code, response or statusText getter. formatError backs the MCP transport error handler (mcp_session_manager.ts:22), so such a throw escapes into an async I/O callback and the error handler becomes the crash. String() also reduces {code: -32601, message: 'Method not found'} to '[object Object]', discarding every field.

Solution: baseMessage now serializes a record with JSON.stringify and truncates it with the existing truncateBody. A plain object therefore surfaces its fields under the same 1000-character bound as an HTTP response body. The String(err) fall-through stays for a value with no serializable own fields, so a class instance with a useful toString is unchanged. One try/catch at the exported boundary returns <unstringifiable value>, mirroring safeJsonSerialize in core/src/telemetry/tracing.ts.

Scope notes:

• The guard sits on formatError, not on baseMessage. Every read that can throw is then covered, including the getter reads in extractHttpDetails and the cause read in formatErrorRecursive, which both run before baseMessage. A guard inside baseMessage would leave those uncovered and the documented promise still false.
• formatError is internal. It is not exported from core/src/index.ts or core/src/common.ts. No export, signature or type changed.
• Collision check: I listed all 300 open PRs on this fork and found none touching core/src/utils/error_utils.ts or formatError. I also read the file lists of the four MCP-adjacent PRs (Feat: add an optional headerProvider to MCPToolset #665, Fix: declare @modelcontextprotocol/sdk in the workspace root manifest #707, Feat: expose an ADK agent as an MCP server (toMcpServer) #580, Feat: port ToolboxToolset from adk-python (MCP Toolbox for Databases) #583); none touch it.

Testing Plan

Unit Tests:

• I have added or updated unit tests for my change.
• All unit tests pass locally.

Ten cases added to core/test/utils/error_utils_test.ts. All 27 existing cases are unedited and still pass: 37 passed.

npx vitest run --project unit:core core/test/utils/error_utils_test.ts

Coverage of core/src/utils/error_utils.ts: 100% of statements, branches, functions and lines.

Proof each test can fail. I ran four mutations and recorded the failures.

Mutation A — revert baseMessage to its original three-line body:

New case	Failure
plain object surfaces its fields	expected '[object Object]' to contain 'errorCode'
circular object	expected '[object Object]' to be '<unstringifiable value>'
BigInt field	expected '[object Object]' to be '<unstringifiable value>'
null-prototype object	expected '<unstringifiable value>' to contain 'no prototype'
oversized object truncated	expected '[object Object]' to contain '... [truncated]'

Mutation B — delete the try/catch from formatError:

New case	Failure
throwing toString	expected [Function] to not throw an error but 'Error: toString exploded' was thrown
throwing getter	expected [Function] to not throw an error but 'Error: getter exploded' was thrown
circular object	expected [Function] to not throw an error but 'TypeError: Converting circular structure...' was thrown
BigInt field	Do not know how to serialize a BigInt

Mutation C — drop the typeof json === 'string' guard: toJSON returns nothing fails with expected '<unstringifiable value>' to be '[object Object]'.

Mutation D — drop the json !== '{}' guard: custom toString fails with expected '{}' to be 'ErrorLike: connection reset', and throwing toString fails with expected '{}' to be '<unstringifiable value>'.

The tenth case, formatError(42), survives all four. It is a regression guard for the primitive path, not a failing-first test.

Manual End-to-End (E2E) Tests:

Not applicable. The change is one utility module, and the unit test file drives it through the public formatError. To reproduce the old behaviour in plain node:

node -e "console.log(String(Object.create(null)))"                      # throws TypeError
node -e "console.log(String({code:-32601,message:'Method not found'}))" # [object Object]

Other checks on the pushed commit:

• npx eslint core/src/utils/error_utils.ts core/test/utils/error_utils_test.ts — clean.
• npx prettier --check on both files — clean.
• npm run ts:check reports 292 errors on this branch and 292 on main. None are in either file I touched.

Checklist

• I have read the CONTRIBUTING.md document.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.