Commit Message: Reorder proto validation error message
Additional Description: As discussed in #38064, the error message is currently often hard to interpret due to the ordering being "high-level low-level high-level" rather than the usual high-to-low ordering error messages have, leading to confusing adjacencies. This is a minimal reordering that doesn't break tests and makes the error message easier to read, for example

Proto constraint validation failed (HttpProtocolOptionsValidationError.ExplicitHttpConfig: embedded message failed validation | caused by field: "protocol_config", reason: is required): explicit_http_config

becomes

explicit_http_config: Proto constraint validation failed (HttpProtocolOptionsValidationError.ExplicitHttpConfig: embedded message failed validation | caused by field: "protocol_config", reason: is required)

Risk Level: Very low it's just an error string.
Testing: Existing tests still pass.
Docs Changes: n/a
Release Notes: n/a
Platform Specific Features: n/a

Request variables

Key	Value
ref	26a6d8c
sha	acdeeef
pr	38089
base-sha	884691f
actor	@ravenblackx
message	Reorder proto validation error message...
started	1737139251.089718
target-branch	main
trusted	false

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