Support configurable TXT record length via TEST_DNS_TXT_LEN (improved acme-dns compatibility)

[kubax]

Summary

This PR adds support for a configurable TXT record length during DNS API testing.
Some DNS systems—such as acme-dns—require TXT values of an exact length (e.g., 43 characters). Until now, acmetest always generated TXT records with a fixed size, making it impossible to properly test systems with strict length requirements.

This change introduces a flexible mechanism that allows the TXT size to be adjusted via a GitHub secret (TEST_DNS_TXT_LEN), while still keeping a safe default.

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What This PR Changes

1. Adds optional TEST_DNS_TXT_LEN secret

• Stored as a GitHub secret
• If set, acmetest generates TXT values with exactly that length
• If not set, a default of 43 characters is used (chosen for acme-dns compatibility)
• Minimum allowed length is 28 characters (to preserve the structure of the original prefix)

2. TXT values are generated from a truly random source

• Uses /dev/urandom or openssl rand depending on platform
• Repeats the random string until long enough, then truncates precisely
• Ensures each test run produces a valid, length-correct value

3. The setting is propagated to all test environments

Including:

• Docker
• macOS
• Windows/Cygwin
• FreeBSD / OpenBSD / NetBSD
• DragonFlyBSD, Solaris, OmniOS

Every environment behaves consistently with the same TXT length configuration.

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Why This Is Useful

The primary motivation is better compatibility with acme-dns, which requires TXT values of a specific length.
This PR makes it possible to:

• Test acme-dns in GitHub Actions
• Adjust TXT length easily for development and debugging
• Change the default later if necessary

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Backward Compatibility

• If TEST_DNS_TXT_LEN is not provided, acmetest defaults to 43 characters
• Existing DNS tests should continue to work as before
• The minimum length safeguard (28+) prevents invalid test values

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Summary

This PR introduces optional configurability for TXT record length in DNS API testing, enabling proper support for acme-dns and other systems that may require fixed-length TXT values.
It preserves compatibility with existing workflows while giving maintainers and developers more control over test behavior.

fixes: #33