Fix example secrets for credential fields without a length

[AJaccP]

Overview

ExampleSecretFromComposition sizes the random part of an example secret as Length - len(prefix) - len(suffix), which assumes Length is always set. It's optional in ValueComposition, and correctly left unset for values that vary in length. With no Length, a field without a prefix produced an empty value, and a field with a prefix produced a negative length that panicked in make([]byte, n). On current main, <plugin>/example-secrets prints blank values for 14 plugins (e.g. Snowflake) and crashes for 11 (e.g. PyPI).

When Length is unset, the value is now built additively: prefix, 24 random characters, then the EXAMPLE suffix when the charset has letters. The random part no longer depends on the prefix, so a long prefix can't consume it. When Length is set, output is unchanged except at the edges: EXAMPLE is only added if at least one random character still fits, and the random part is clamped at zero so a Length shorter than the prefix can't panic. All 61 existing fields that declare a Length still produce exactly that length.

Type of change

• Created a new plugin
• Improved an existing plugin
• Fixed a bug in an existing plugin
• Improved contributor utilities or experience

Related Issue(s)

• Resolves: Blank example credential when the field doesn't have a length defined #215

How To Test

Unit tests, including regression tests for the reported bug:

go test ./sdk/plugintest/ -v

End to end with the contributor tooling. These printed blank values or panicked before this change, and should now print a full value for every field:

make snowflake/example-secrets
make pypi/example-secrets

These declare a fixed Length and should print values of exactly that length, as before:

make github/example-secrets    # 40 characters
make aws/example-secrets       # 20 and 40 characters

Changelog

<plugin>/example-secrets now generates example values for credential fields without a Length, instead of printing blanks or crashing.

[JillRegan]

LGTM!!