docs: linear retry strategy and retry helpers

[yaythomas]

Documents the linear retry strategy helper and the retry helper for
durable operations that cannot live inside a step.

Changes to docs/sdk-reference/error-handling/retries.md

• Restructure Configure a retry strategy so Exponential backoff and
  Linear backoff are sibling subsections, each with its own walkthrough,
  config signature, and delay-calculation block.
• Add Retry a durable operation covering the retry helper:
  TypeScript withRetry, Python with_retry, Java
  DurableContext.withRetry / withRetryAsync. Explains when to use it
  instead of a step retry strategy (for invoke, waitForCallback,
  waitForCondition, etc., which cannot be inside a step).
• Update Retry presets: add retryPresets.linear (TypeScript),
  RetryStrategies.Presets.LINEAR (Java), and
  RetryPresets.linear() / RetryPresets.fixed(interval) (Python).

New examples

• linear-retry-strategy.{ts,py,java} — configures a linear strategy
  on a step.
• linear-retry-strategy-config-signature.{ts,py,java} — config shape
  reference, mirroring the existing retry-strategy-config-signature.*.
• with-retry-helper.{ts,py,java} — wraps an invoke with the retry
  helper.

The existing retry-presets.{ts,py,java} examples are updated to
demonstrate the new linear preset (and Python fixed).

Source verification

All signatures verified against origin/main of each SDK repo. One
correction relative to the issue text:

• The TypeScript helper is withRetry, not retryOperation. It is
  exported from @aws/durable-execution-sdk-js (see
  packages/aws-durable-execution-sdk-js/src/utils/with-retry/index.ts
  and the package index.ts re-export).
• Java RetryStrategies.linearBackoff (the existing factory name), not
  RetryStrategies.linear.

Verification

• mdformat docs/sdk-reference/error-handling/retries.md is idempotent.
• zensical build --clean finishes cleanly: No issues found.

Closes #155
Closes #209