fix(bedrock-agent): make flow and prompt operations work and match the model

[vieiralucas]

Summary

Found while checking Bedrock Agent response shapes after the ARN partition campaign.

Flows were broken over the wire. Every flow handler read flowId from the body, but the router supplies flowIdentifier (and aliasIdentifier), so GetFlow/UpdateFlow/DeleteFlow/PrepareFlow and every flow version and alias op always failed with a missing-field error. They now read the modeled names and accept the ID or the (URL-encoded) ARN.

Responses match the Smithy output shapes

• GetFlow/UpdateFlow and GetPrompt/UpdatePrompt return the model's top-level members (id, stored arn, ...), not a {"flow":{"flowId"}} / {"prompt":{"promptId"}} wrapper; customerEncryptionKeyArn and defaultVariant are persisted.
• Flow versions and aliases use the model shapes (alias ARN flow/<id>/alias/<alias-id>); deletes return their modeled id fields.
• PrepareFlow returns Prepared (was the enum name PREPARED; old snapshots normalized on load); creates return 201, PrepareFlow 202; UpdateFlow resets the flow to NotPrepared.

ValidateFlowDefinition was routed to PrepareFlow. It now has its own route and checks structure (missing start/end node, unknown nodes/inputs, true duplicate connections keyed on source/target/output/input, unfulfilled or multiply-connected inputs, cycles, unreachable nodes, duplicate/empty node names) and returns the modeled validations list.

Versions and identifiers

• Flow and prompt version numbers come from a persisted counter, so a deleted number is never reused.
• DeletePrompt validates promptVersion as NumericalVersion (DRAFT/non-numeric -> ValidationException) instead of deleting the whole prompt; GetPrompt/DeletePrompt honor ?promptVersion=; ListPrompts honors promptIdentifier (the prompt's versions).
• Prompt versions capture customerEncryptionKeyArn at creation.
• ARN identifiers are parsed with arn_resource and must match the caller's account, region and partition (and an alias must belong to the flow in the path); foreign ARNs return ResourceNotFound.
• Path labels are percent-decoded once in handle(); tags are keyed by the decoded ARN, and older snapshots' percent-encoded tag keys are re-keyed on load.
• Removed the non-model GetPromptVersion/ListPromptVersions routes (the model reaches versions via GetPrompt/ListPrompts query members).

No surface changes (no SDK/introspection change).

Test plan

• Route-level tests through handle() asserting exact key sets for flows, prompts and validate-definition; version numbering, DeletePrompt DRAFT rejection, ListPrompts by identifier, version encryption key, foreign-ARN rejection, AWS-sample flow validation, duplicate node names, tag re-keying on load. New tests fail without the change.
• cargo test -p fakecloud-bedrock-agent (14), clippy -D warnings, fmt, cargo check --workspace --all-targets.
• Conformance nextest (bedrock-agent, bedrock-agent-runtime) 6/6; probe conformance run --services bedrock-agent,bedrock-agent-runtime: 72/72 and 31/31 ops, 3669/3669 variants.

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Summary by cubic

Fixes Bedrock Agent flow and prompt operations so they work over the wire and return the Smithy model's output shapes. Nearly every flow operation failed because handlers read a flowId body member the router never supplies, and responses carried wrapper objects the model doesn't define.

Bug Fixes

• Flow handlers read the modeled flowIdentifier/aliasIdentifier and accept a bare ID or the URL-encoded ARN.
• Get/Update flow and prompt responses return top-level members (id, stored arn, ...) instead of {"flow":{"flowId"}} / {"prompt":{"promptId"}} wrappers; customerEncryptionKeyArn and defaultVariant are persisted.
• Flow versions and aliases use the model shapes; creates return 201 (PrepareFlow 202 and Prepared, was PREPARED); UpdateFlow resets the flow to NotPrepared.
• ValidateFlowDefinition is now its own route (was routed to PrepareFlow) and returns the modeled validations list.
• DeletePrompt rejects non-numeric promptVersion instead of deleting the whole prompt; GetPrompt/DeletePrompt honor ?promptVersion=; ListPrompts honors promptIdentifier.
• ARN identifiers must name the caller's account, region and partition; foreign ARNs return ResourceNotFound.

Versions and identifiers

• Version numbers come from a persisted per-resource counter, so a deleted number is never reused; prompt versions capture customerEncryptionKeyArn at creation.
• Path labels are percent-decoded once in handle(); tag keys stored percent-encoded are re-keyed on load.
• Removed the non-model GetPromptVersion/ListPromptVersions routes; versions are reached via GetPrompt/ListPrompts query members.

^{Written for commit 6621fb8. Summary will update on new commits.}