feat: configurable idempotency key validation (max length + pattern)

[ashudhanda]

Implements the key-validation half of #2 in the Spring Boot starter.

What changed

Two new configuration properties under avoonce.idempotency.* (IdempotencyProperties):

avoonce:
  idempotency:
    key-pattern: "^[a-fA-F0-9\\-]{36}$"  # optional; disabled by default
    max-key-length: 128                    # default 255

• key-pattern — optional regex the key must fully match (e.g. UUID format, as suggested in the issue). null/blank (the default) disables it, so existing setups are unaffected. The pattern is compiled once in the IdempotencyFilter constructor, not per request.
• max-key-length — keys longer than this are rejected with HTTP 400 before touching the store, preventing abuse via overly long keys. Default 255.
• Blank/whitespace-only keys are now also rejected with 400 — previously an all-spaces header would flow into the state machine as a "real" key.

Validation runs in IdempotencyFilter after the existing null/enforce check and before the state machine is engaged, so an invalid key never acquires a lock or writes a record. The 400 body names the configured header and the specific violation.

Tests

Added to IdempotencyFilterTest:

• blank key → 400, controller never executes
• key over max-key-length → 400; key within the limit → 201 (normal flow)
• key failing key-pattern → 400; UUID-shaped key → 201 (normal flow)

Not in this PR

The second part of the issue — sourcing the key from a query param or JSON-body field instead of a header — is a larger, separable change (it touches CachedBodyHttpServletRequest buffering order and the JAX-RS adapter for parity). Happy to follow up with it in a separate PR if the header-first shape here looks right to you; the same validation would then apply to whichever source is configured.

Refs #2

[raghavgopal]

Implements the key-validation half of #2 in the Spring Boot starter.

What changed

Two new configuration properties under avoonce.idempotency.* (IdempotencyProperties):

avoonce:
  idempotency:
    key-pattern: "^[a-fA-F0-9\\-]{36}$"  # optional; disabled by default
    max-key-length: 128                    # default 255

• key-pattern — optional regex the key must fully match (e.g. UUID format, as suggested in the issue). null/blank (the default) disables it, so existing setups are unaffected. The pattern is compiled once in the IdempotencyFilter constructor, not per request.
• max-key-length — keys longer than this are rejected with HTTP 400 before touching the store, preventing abuse via overly long keys. Default 255.
• Blank/whitespace-only keys are now also rejected with 400 — previously an all-spaces header would flow into the state machine as a "real" key.

Validation runs in IdempotencyFilter after the existing null/enforce check and before the state machine is engaged, so an invalid key never acquires a lock or writes a record. The 400 body names the configured header and the specific violation.

Tests

Added to IdempotencyFilterTest:

• blank key → 400, controller never executes
• key over max-key-length → 400; key within the limit → 201 (normal flow)
• key failing key-pattern → 400; UUID-shaped key → 201 (normal flow)

Not in this PR

The second part of the issue — sourcing the key from a query param or JSON-body field instead of a header — is a larger, separable change (it touches CachedBodyHttpServletRequest buffering order and the JAX-RS adapter for parity). Happy to follow up with it in a separate PR if the header-first shape here looks right to you; the same validation would then apply to whichever source is configured.

Refs #2

Thanks for the PR, I will get back after reviewing.