fix(rfc9111): implement must-understand and no-store interaction per RFC 9111 Section 5.2.2.3

[k1LoW]

According to RFC 9111 Section 5.2.2.3, when a cache that implements the
must-understand directive receives a response that includes it, the cache
SHOULD ignore the no-store directive if it understands and implements the
status code's caching requirements.

This change modifies the Storable() method to ignore no-store when:

• must-understand directive is present
• AND the cache understands the response status code

Added test cases to verify the behavior:

• Status 200 with must-understand + no-store -> cacheable
• Status 418 with must-understand + no-store -> cacheable
• Status 299 (unknown) with must-understand + no-store -> not cacheable

[github-actions]

Code Metrics Report

	main (cf0ad24)	#98 (a91230c)	+/-
Coverage	61.7%	62.0%	+0.2%
Code to Test Ratio	1:2.3	1:2.4	+0.0
Test Execution Time	1m0s	1m2s	+2s

Details

  |                     | main (cf0ad24) | #98 (a91230c) |  +/-  |
  |---------------------|----------------|---------------|-------|
+ | Coverage            |          61.7% |         62.0% | +0.2% |
  |   Files             |              8 |             8 |     0 |
  |   Lines             |            513 |           514 |    +1 |
+ |   Covered           |            317 |           319 |    +2 |
+ | Code to Test Ratio  |          1:2.3 |         1:2.4 |  +0.0 |
  |   Code              |            853 |           854 |    +1 |
+ |   Test              |           2029 |          2077 |   +48 |
- | Test Execution Time |           1m0s |          1m2s |   +2s |

Code coverage of files in pull request scope (78.5% → 79.0%)

Files	Coverage	+/-	Status
rfc9111/directive.go	67.1%	+1.4%	affected
rfc9111/shared.go	83.3%	+0.0%	modified

---

Reported by octocov