Name the port and the cause when a port name is rejected

[dv-picknik]

CreatePort, behind InputPort, OutputPort and BidirectionalPort, throws one fixed message whenever IsAllowedPortName fails. It never names the port, and it blames the first character even when the cause is a forbidden character. InputPort<double>("target.x") compiles and then throws at registerNodeType with:

The name of a port must not be `name` or `ID` and must start with an alphabetic character. Underscore is reserved.

This names the port and the check that failed:

Port	Error
target.x	Port name 'target.x' contains forbidden character '.'
goal pose	Port name 'goal pose' contains forbidden character ' '
_foo	Port name '_foo' must start with an alphabetic character, since a leading underscore is reserved
name	Port name 'name' is a reserved attribute name

ThrowInvalidPortName builds the message in src/basic_types.cpp, next to IsAllowedPortName. It runs the same checks in the same order, so the reason it names is the one that failed. Because it lives in the library rather than the CreatePort template, the error path is no longer compiled into every translation unit that declares a port. It adds one exported symbol and changes no existing one.

Testing

• pixi run build && pixi run test, 531/531.
• NameValidation.CreatePort_ErrorNamesTheCause fails on master and passes here. For each failure path, including control characters, it checks that the message names the port and the right reason, and not a wrong one. Three inputs fail two checks at once, _a.b, .x and 1.5, which pins the check order.
• pre-commit run is clean. clangd-21, installed per CONTRIBUTORS_GUIDE.md in an Ubuntu 24.04 container, reports All checks completed, 0 errors on both changed source files.

Related to #1215, where the same two rules disagree about a leading underscore.

Co-authored by Claude Opus 5.5