[py systems] Change how ValueProducer callbacks work

[jwnimmer-tri]

(1) Add better error reporting in case the user's function is incorrectly implemented or otherwise misbehaves.

(2) Don't rely on pybind11 to cast the result of allocate() to a unique_ptr (this is an RLG/pybind11-specific feature).

Towards #21968.

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This change is 

[sherm1]

Do you want to assign a feature reviewer for this, Jeremy?

[jwnimmer-tri]

Sadly, I think we need to pile another feature review on +@rpoyner-tri, unless @EricCousineau-TRI wants to jump in? This can wait until next week, it's not blocking anything.

[EricCousineau-TRI]

feature, can also serve as platform if desired - thanks!

Reviewed 8 of 8 files at r1.
Reviewable status: LGTM missing from assignee rpoyner-tri(platform), needs at least two assigned reviewers

[jwnimmer-tri]

Thanks! I think extra eyes are good in this case, so +@SeanCurtis-TRI for platform per schedule, please.

[SeanCurtis-TRI]

Reviewed 8 of 8 files at r1, all commit messages.
Reviewable status: 1 unresolved discussion

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bindings/pydrake/systems/value_producer_pybind.cc line 6 at r1 (raw file):

namespace pydrake {

// The C++ framework uses ValueProducer::AllocateCallback to allocate storage

nit: This is copied verbatim from the header. Do you want it both places?

[SeanCurtis-TRI]

Reviewed 1 of 1 files at r2, all commit messages.
Reviewable status: 1 unresolved discussion