Trim the key of a map flag when TrimSpace is set

[youdie006]

What type of PR is this?

• bug

What this PR does / why we need it:

StringConfig.TrimSpace reaches only half of each pair in a map flag. MapBase.Set routes the value through the configured parser at flag_map_impl.go:84 but writes the key into the dict raw at :87, so the key never sees the config:

--m " a = 1 "   TrimSpace: true    before  map[" a ":"1"]     after  map["a":"1"]
--m " a = 1 "   TrimSpace: false   before  map[" a ":" 1 "]   after  map[" a ":" 1 "]
--f "a, b"      TrimSpace: true    StringSliceFlag gives ["a","b"], both trimmed

cmd.StringMap("m")["b"] misses on --m "a=1, b=2" because the stored key is " b".

SliceBase has routed every element through the parser since #2171; MapBase is the one place a StringConfig-carrying string reaches storage without passing the config.

Which issue(s) this PR fixes:

None filed. Follows #2171 and #2425, which fixed the value side of the same seam.

Release Notes

Map flags now apply `StringConfig.TrimSpace` to the key as well as the value.

Verification

Base 9b981a0.

row	flag_map_impl.go md5	subtests run	result
this PR	7031d44c5d	3	ok
base	f33bcad660	3	fails trim_enabled
revert the guarded trim	c3186d07eb	3	fails trim_enabled
trim the key unconditionally	caa059492c	3	fails trim_disabled

The two mutants fail on different subtests, so the test pins both that the key is trimmed under TrimSpace: true and that it is left alone under TrimSpace: false.

make vet, make test, make v3diff and make generate all clean; gofmt -l . empty; no exported API change and no godoc drift. make check-binary-size and the gfmrun examples were not run here.

One thing worth your call: reading the config in Create with any(c).(StringConfig) mirrors the reflect.TypeOf(t).Kind() approach SliceBase already uses, but it is still a type assertion inside a generic. If you would rather thread it differently, the test and the behaviour survive either shape. StringMapFlag is currently the only map flag the library ships, so this does not affect any other element type.

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Disclosure: written with the help of Claude (an AI assistant). Every number above is from runs on my machine.