MAINT: add missing space in warning and error messages (#32405)

[charris]

Backport of #32405.

PR summary

Five user-facing messages run two sentences together because adjacent string
literals are concatenated without a trailing space:

>>> np.sum(i for i in range(3))
TypeError: Calling np.sum(generator) is deprecated.Use np.sum(...)

This is the counterpart of b8c885d ("DOC: add space between words across
lines"), which fixed the format 3.0 warning in what is now _format_impl.py
(then numpy/lib/format.py); the format 2.0 warning five lines above it still
lacks the space. The other sites are in _core/fromnumeric.py,
ma/mrecords.py, and lib/_twodim_base_impl.py (×2).

One space per site, no logic changed. These are all the occurrences I found in
the package — similar-looking joins that are intentional (the regex fragments in
_core/_internal.py, the C macro template in code_generators/genapi.py,
fragments whose continuation begins with \n) were left alone. No test asserts
across any of these boundaries.

First time committer introduction

Hi, I work in Python for data analysis and data science, and lately I've been
building LLM agents on the side. I was reading through the error-handling code
of another library when I noticed this string-concatenation pattern, and since
NumPy is the library I use most, I checked whether it had the same issue — it
did, in five places. This is my first contribution to NumPy.

AI Disclosure

I used an AI assistant (Claude) to help scan for these sites and to draft and
polish this description. I ran and verified everything myself: all five messages
were reproduced locally on NumPy 2.5.2, the excluded cases were reviewed one by
one, and the 2019 precedent was confirmed in the git history. The change is five
single-space insertions with no logic change.