Harden ramp.v1 identifier field rules to match ramp.admin.v1

[KonstantinMirin]

Context

ramp.admin.v1 (added in the admin RPC surface work, #18) bounds its identifier fields — e.g. tenant_id [(buf.validate.field).string = {min_len: 1, max_len: 255}]. The equivalent identifiers in ramp.v1 are unconstrained:

• UsageReport.transaction_id (proto/ramp/v1/ramp.proto:2104) — bare
• UsageReportResponse.report_id (proto/ramp/v1/ramp.proto:2208) — bare, and load-bearing (dispute-chain reference)
• tenant_id at proto/ramp/v1/ramp.proto:1929, 2031, 2047 — bare

The admin package's stricter rules are the better convention. This issue tracks bringing the ramp.v1 identifiers in line (min_len, a sane max_len, and a token pattern where appropriate).

Why it's separate from the admin PR

This touches the agent hot-path contract: the rules regenerate the Zod/Pydantic types and can newly-reject inputs that validate today. It needs its own changelog entry and a compatibility check against existing corpus/doc examples, so it shouldn't ride the admin change.

Scope

• Audit all ramp.v1 identifier fields for missing string bounds.
• Apply rules consistent with ramp.admin.v1.
• Regenerate clients; confirm the conformance corpus + doc examples still pass (expect additive below_min/pattern mutants, not changed cases).
• Changelog entry under the proto CHANGELOG.