CRED-2149: Add PAT auth support to Java API client

[tausman]

Why

Adds Personal Access Token (PAT) authentication support to the Java API client. PATs can be used via the Authorization: Bearer <PAT> header, providing an alternative authentication method alongside API key + app key auth.

Jira: CRED-2149

Summary

• Env var: DD_BEARER_TOKEN -- when set, the client sends Authorization: Bearer <token> on every request
• Behavior: Auth methods are NOT mutually exclusive. All configured headers are sent simultaneously (DD-API-KEY, DD-APPLICATION-KEY, Authorization: Bearer), and the server uses whichever is valid
• Generator template (ApiClient.j2): Added spec-driven support for type: http security schemes. Reads x-env-name from the OpenAPI spec for env var configuration. Iterates over security schemes rather than hardcoding names
• ApiClient.java (generated): Registers bearerAuth in the authentication map, reads DD_BEARER_TOKEN from the environment, and applies bearer auth alongside any other configured auth headers
• HttpBearerAuth (pre-existing class): Applies Authorization: Bearer <token> to request headers
• HttpBearerAuthTest: Unit tests for token get/set, header application, null-token no-op, client registration, and client set/get round-trip

Changes

• .generator/src/generator/templates/ApiClient.j2 -- http type handler in security scheme loop, spec-driven env var reading via x-env-name, bearer auth applied alongside other auth in updateParamsForAuth()
• src/main/java/com/datadog/api/client/ApiClient.java -- generated output with bearerAuth registration, DD_BEARER_TOKEN env var reading, non-exclusive auth header application
• src/test/java/com/datadog/api/client/auth/HttpBearerAuthTest.java -- 5 unit tests

Test plan

• Unit tests pass (5/5 in HttpBearerAuthTest)
• E2E verification against staging (api.datad0g.com/api/v2/current_user) with Authorization: Bearer header returns 200
• Confirmed all configured auth headers are sent independently -- no suppression or mutual exclusivity
• No secrets in committed files

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PR Stack

API Client Libraries

• Go: CRED-2147: Add PAT auth support to Go API client datadog-api-client-go#3764 — CRED-2147: Add PAT auth support to Go API client
• Python: CRED-2146: Add PAT auth support to Python API client datadog-api-client-python#3243 — CRED-2146: Add PAT auth support to Python API client
• TypeScript: CRED-2148: Add PAT auth support to TypeScript API client datadog-api-client-typescript#3588 — CRED-2148: Add PAT auth support to TypeScript API client
• Java: CRED-2149: Add PAT auth support to Java API client #3555 — CRED-2149: Add PAT auth support to Java API client
• Rust: CRED-2150: Add PAT auth support to Rust API client datadog-api-client-rust#1310 — CRED-2150: Add PAT auth support to Rust API client
• Ruby: CRED-2151: Add PAT auth support to Ruby API client datadog-api-client-ruby#3058 — CRED-2151: Add PAT auth support to Ruby API client

OpenAPI Spec Changes

• https://github.com/DataDog/datadog-api-spec/pull/5166 — Add bearerAuth to global default security for PAT support
• https://github.com/DataDog/datadog-api-spec/pull/5164 — Add PAT/SAT management API endpoints (CRED-2275)