fix(base64): use standard RFC 4648 alphabet on the text path (#87)

[fredbi]

strfmt.Base64 maps to OpenAPI format: byte, which OAI v2 defines as unqualified "base64 encoded characters".

Per RFC 4648, unqualified "base64" is the standard alphabet (section 4, +/); base64url (section 5, -_) is a separately named encoding the RFC says must not be called "base64".

Eight of the nine Base64 serialization paths already used StdEncoding (String, MarshalJSON/UnmarshalJSON, Scan/Value, MarshalBSON/UnmarshalBSON).

The lone outlier was the encoding.TextMarshaler/TextUnmarshaler pair, which used URLEncoding since PR #57.

The result: the same Base64 value serialized to a query parameter (text) versus a JSON body encoded differently whenever the payload hit alphabet index 62/63, breaking opaque tokens that cross boundaries.

Route every path through a single seam, base64Encoding = base64.StdEncoding, so the whole type now uses standard base64 and there is exactly one place to introduce a URL-safe variant later (see the type-parameterization work). The misleading "URLEncoding alphabet" godoc is corrected.

Behavioral change (patch-level spec-conformance fix): the text path now emits and accepts standard base64. base64url text that previously round-tripped via UnmarshalText will now be rejected. In practice this was rarely hit because consumers marshal via JSON even for parameters.

• fixes Base64 type states that it represents URL encoded string but in JSON, it uses standard base64 encoding  #87

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Please select: 🆕 New feature or enhancement|🔧 Bug fix'|📃 Documentation update

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• I have signed all my commits with my name and email (see DCO. This does not require a PGP-signed commit
• I have rebased and squashed my work, so only one commit remains
• I have added tests to cover my changes.
• I have properly enriched go doc comments in code.
• I have properly documented any breaking change.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.81%. Comparing base (7f8ff38) to head (a7a118b).
✅ All tests successful. No failed tests found.

Additional details and impacted files

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  Coverage   87.81%   87.81%           
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  Files          18       18           
  Lines        2471     2471           
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  Hits         2170     2170           
  Misses        208      208           
  Partials       93       93           

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